What Is SMTP and How Does It Work? | mySMTP Blog<\/title>\n<style>\n @import url('https:\/\/fonts.googleapis.com\/css2?family=Inter:wght@400;500;600;700&display=swap');\n\n * { margin: 0; padding: 0; box-sizing: border-box; }\n\n body {\n font-family: 'Inter', system-ui, -apple-system, sans-serif;\n background: #f5f5f5;\n padding: 20px;\n color: #1a1a1a;\n line-height: 1.6;\n }\n\n .container {\n max-width: 960px;\n margin: 0 auto;\n background: white;\n border-radius: 12px;\n overflow: hidden;\n box-shadow: 0 2px 8px rgba(0,0,0,0.08);\n }\n\n \/* \u2500\u2500 HEADER \u2500\u2500 *\/\n .header { background: #16a34a; color: white; padding: 44px 40px; }\n .header .eyebrow { font-size: 0.78rem; font-weight: 600; letter-spacing: 0.06em; text-transform: uppercase; opacity: 0.85; margin-bottom: 12px; }\n .header h1 { font-size: 2.1rem; margin-bottom: 12px; font-weight: 700; letter-spacing: -0.5px; line-height: 1.25; }\n .header .subtitle { font-size: 0.98rem; opacity: 0.92; font-weight: 400; max-width: 680px; }\n\n \/* \u2500\u2500 SECTION DIVIDER (matches toolbox headers) \u2500\u2500 *\/\n .section-divider {\n display: flex; align-items: center; gap: 16px;\n padding: 0 40px; background: #f9fafb;\n border-top: 1px solid #e5e7eb; border-bottom: 1px solid #e5e7eb;\n }\n .section-divider-label {\n display: flex; align-items: center; gap: 8px; padding: 14px 0;\n font-size: 0.82rem; font-weight: 700; letter-spacing: 0.04em; text-transform: uppercase;\n color: #374151; white-space: nowrap;\n }\n .section-divider-label svg { color: #16a34a; flex-shrink: 0; }\n .section-divider-line { flex: 1; height: 1px; background: #e5e7eb; }\n\n \/* \u2500\u2500 SECTIONS \u2500\u2500 *\/\n .section { padding: 30px 40px 6px; }\n .section:last-of-type { padding-bottom: 30px; }\n\n h3 { font-size: 1.05rem; font-weight: 600; color: #1a1a1a; margin: 22px 0 8px; }\n h3:first-child { margin-top: 0; }\n p { color: #374151; font-size: 0.96rem; margin-bottom: 14px; }\n p:last-child { margin-bottom: 0; }\n strong { color: #1a1a1a; font-weight: 600; }\n\n .mono {\n font-family: 'Courier New', monospace; background: #f3f4f6; color: #166534;\n padding: 2px 7px; border-radius: 4px; font-size: 0.88em;\n }\n\n .flow-box {\n font-family: 'Courier New', monospace; font-size: 0.9rem; color: #166534;\n background: #f0fdf4; border: 1px solid #bbf7d0; border-radius: 8px;\n padding: 12px 16px; margin: 10px 0; display: inline-block;\n }\n .flow-stack { margin: 10px 0 16px; }\n .flow-stack .flow-box { display: block; margin: 0 0 8px; }\n .flow-stack .flow-box:last-child { margin-bottom: 0; }\n\n \/* \u2500\u2500 CARD-STYLE BULLET LISTS (matches toolbox list treatment) \u2500\u2500 *\/\n ul.card-list {\n list-style: none; margin: 0 0 16px; padding: 0;\n display: grid; grid-template-columns: 1fr 1fr; gap: 8px;\n }\n ul.card-list.single-col { grid-template-columns: 1fr; }\n ul.card-list li {\n display: flex; align-items: center; gap: 10px;\n background: #f9fafb; border: 1px solid #e5e7eb; border-radius: 8px;\n padding: 9px 12px; font-size: 0.88rem; color: #374151;\n }\n ul.card-list li svg { color: #16a34a; flex-shrink: 0; }\n @media (max-width: 560px) { ul.card-list { grid-template-columns: 1fr; } }\n\n \/* \u2500\u2500 GENERIC CTA BOX \u2500\u2500 *\/\n .cta-box {\n background: #f0fdf4; border: 1px solid #bbf7d0; border-radius: 10px;\n padding: 26px 28px; margin: 24px 0; margin-top: 22px;\n display: flex; align-items: center; justify-content: space-between; gap: 20px; flex-wrap: wrap;\n }\n .cta-box .cta-icon {\n width: 42px; height: 42px; border-radius: 10px; background: #16a34a; color: white;\n display: flex; align-items: center; justify-content: center; flex-shrink: 0;\n }\n .cta-content { display: flex; align-items: center; gap: 16px; flex: 1; min-width: 240px; }\n .cta-text h4 { font-size: 1.02rem; font-weight: 600; color: #166534; margin-bottom: 4px; }\n .cta-text p { font-size: 0.86rem; color: #4b5563; margin: 0; }\n .cta-btn {\n padding: 12px 24px; background: #16a34a; color: white !important; border: none;\n border-radius: 8px; font-size: 0.9rem; font-weight: 600; cursor: pointer;\n text-decoration: none; white-space: nowrap; font-family: 'Inter', sans-serif;\n display: inline-flex; align-items: center; gap: 8px; transition: all 0.2s; flex-shrink: 0;\n }\n .cta-btn:hover { background: #15803d; transform: translateY(-1px); box-shadow: 0 4px 6px rgba(0,0,0,0.1); }\n\n \/* \u2500\u2500 FEATURED TOOL CTA (matches the free email test screenshot) \u2500\u2500 *\/\n .featured-cta {\n position: relative; overflow: hidden;\n background:\n radial-gradient(circle, #d1fae5 1px, transparent 1px) 0 0\/16px 16px,\n linear-gradient(135deg, #f0fdf4 0%, #ecfdf5 100%);\n border: 1px solid #bbf7d0; border-radius: 12px;\n padding: 34px; margin: 24px 0; margin-top: 22px;\n display: flex; align-items: center; justify-content: space-between; gap: 30px; flex-wrap: wrap;\n }\n .featured-left { flex: 1; min-width: 260px; }\n .featured-badge {\n display: inline-flex; align-items: center; gap: 6px;\n background: #16a34a; color: white; font-size: 0.72rem; font-weight: 700;\n letter-spacing: 0.03em; padding: 5px 11px; border-radius: 100px; margin-right: 10px;\n }\n .featured-eyebrow { font-size: 0.75rem; color: #6b7280; letter-spacing: 0.04em; vertical-align: middle; }\n .featured-live { display: inline-flex; align-items: center; gap: 5px; font-size: 0.75rem; color: #16a34a; font-weight: 600; margin-left: 8px; }\n .featured-live::before { content: ''; width: 6px; height: 6px; border-radius: 50%; background: #16a34a; display: inline-block; }\n .featured-left h3 { font-size: 1.5rem; font-weight: 700; margin: 14px 0 10px; letter-spacing: -0.4px; }\n .featured-left h3 .accent { color: #16a34a; }\n .featured-left > p { font-size: 0.92rem; color: #4b5563; max-width: 460px; margin-bottom: 16px; }\n .featured-checks { display: flex; flex-wrap: wrap; gap: 14px; margin-bottom: 20px; }\n .featured-check { display: flex; align-items: center; gap: 6px; font-size: 0.82rem; color: #374151; }\n .featured-check svg { color: #16a34a; flex-shrink: 0; }\n .featured-actions { display: flex; align-items: center; gap: 20px; flex-wrap: wrap; }\n .featured-actions a.how-it-works { font-size: 0.9rem; color: #1a1a1a; font-weight: 500; text-decoration: none; display: inline-flex; align-items: center; gap: 5px; }\n\n .mock-browser {\n width: 300px; flex-shrink: 0; background: white; border-radius: 10px;\n box-shadow: 0 8px 24px rgba(0,0,0,0.1); border: 1px solid #e5e7eb; overflow: hidden;\n }\n .mock-browser-bar { display: flex; align-items: center; gap: 6px; padding: 10px 14px; border-bottom: 1px solid #f0f0f0; }\n .mock-dot { width: 8px; height: 8px; border-radius: 50%; }\n .mock-dot.red { background: #ef4444; } .mock-dot.gray { background: #d1d5db; } .mock-dot.green { background: #22c55e; }\n .mock-browser-title { flex: 1; text-align: center; font-size: 0.65rem; color: #9ca3af; letter-spacing: 0.04em; }\n .mock-browser-body { padding: 16px; }\n .mock-label { font-size: 0.72rem; color: #6b7280; margin-bottom: 6px; }\n .mock-input { font-family: 'Courier New', monospace; font-size: 0.72rem; background: #f9fafb; border: 1px solid #e5e7eb; border-radius: 6px; padding: 8px 10px; margin-bottom: 14px; color: #374151; }\n .mock-row { display: flex; align-items: center; justify-content: space-between; padding: 8px 0; border-top: 1px solid #f3f4f6; font-size: 0.78rem; color: #374151; }\n .mock-row:first-of-type { border-top: none; }\n .mock-pass { color: #16a34a; font-weight: 600; display: inline-flex; align-items: center; gap: 4px; }\n\n \/* \u2500\u2500 FAQ \u2500\u2500 *\/\n .faq-item { border-bottom: 1px solid #e5e7eb; padding: 18px 0; }\n .faq-item:first-child { padding-top: 0; }\n .faq-item:last-child { border-bottom: none; }\n .faq-item h3 { margin: 0 0 6px; }\n .faq-item p { margin: 0; }\n\n \/* \u2500\u2500 ABOUT FOOTER \u2500\u2500 *\/\n .about-box { background: #f9fafb; border-top: 1px solid #e5e7eb; padding: 34px 40px; }\n .about-box p { color: #4b5563; }\n\n @media (max-width: 640px) {\n .header, .section, .about-box, .section-divider { padding-left: 20px; padding-right: 20px; }\n .featured-cta { padding: 24px; }\n .mock-browser { width: 100%; }\n }\n<\/style>\n<\/head>\n<body>\n<div class=\"container\">\n\n \n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M4 4h16v16H4z\"><\/path><path d=\"M4 8h16\"><\/path><\/svg>Overview<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>In simple terms:<\/p>\n <div class=\"flow-box\">Sender \u2192 SMTP server \u2192 recipient's mail server \u2192 inbox<\/div>\n <p>Every time a webshop sends an order confirmation, a website sends a password reset, or a business sends a marketing email, SMTP is part of the delivery process.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M9.09 9a3 3 0 015.83 1c0 2-3 2-3 4\"><\/path><path d=\"M12 17h.01\"><\/path><\/svg>What Does SMTP Stand For?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>SMTP stands for Simple Mail Transfer Protocol.<\/p>\n <p>It is a standard set of rules that allows email systems to communicate with each other when sending and transferring email.<\/p>\n <p>SMTP has existed since the early days of the internet, but it remains the foundation of email delivery today.<\/p>\n <p>Even when an application sends email through an email API, SMTP is normally involved later when the provider transfers the message to the recipient's mail server.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><rect x=\"3\" y=\"4\" width=\"18\" height=\"7\" rx=\"1.5\"><\/rect><rect x=\"3\" y=\"13\" width=\"18\" height=\"7\" rx=\"1.5\"><\/rect><path d=\"M7 7.5h.01M7 16.5h.01\"><\/path><\/svg>What Is an SMTP Server?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>An SMTP server is a mail server that accepts, processes, queues and transfers outgoing email.<\/p>\n <p>Think of it as a specialized delivery system for email.<\/p>\n <p>Your website, application or email program hands the message to an SMTP server. The SMTP server then determines where the message needs to go and attempts to deliver it to the recipient's mail provider.<\/p>\n <p>For example:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">Webshop \u2192 SMTP server \u2192 Gmail \u2192 customer<\/div>\n <\/div>\n <p>or:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">CRM \u2192 SMTP server \u2192 Microsoft 365 \u2192 customer<\/div>\n <\/div>\n <p>The SMTP server handles the technical communication required to move the email between these systems.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M13 2L3 14h7l-1 8 10-12h-7l1-8z\"><\/path><\/svg>How Does SMTP Work?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>When you click Send, several things happen in just a few seconds.<\/p>\n\n <h3>1. The application connects to an SMTP server<\/h3>\n <p>The sending system could be Outlook, WordPress, a webshop, CRM, accounting software, SaaS platform, IoT device or custom application.<\/p>\n <p>It connects to an SMTP server using a hostname such as:<\/p>\n <p><span class=\"mono\">smtp.example.com<\/span><\/p>\n <p>Applications typically connect using SMTP port 587, 465 or sometimes 2525.<\/p>\n\n <h3>2. The SMTP connection starts<\/h3>\n <p>The sending application introduces itself using an SMTP command such as:<\/p>\n <p><span class=\"mono\">EHLO<\/span><\/p>\n <p>The SMTP server responds with the features it supports, which can include authentication and TLS encryption.<\/p>\n <p>If authentication is required, the application proves that it is authorized to use the SMTP server.<\/p>\n\n <h3>3. SMTP identifies the sender<\/h3>\n <p>The sending system provides the envelope sender using:<\/p>\n <p><span class=\"mono\">MAIL FROM:<sender@example.com><\/span><\/p>\n <p>This address is part of the SMTP envelope.<\/p>\n <p>The SMTP envelope contains the routing information mail servers need to deliver the message.<\/p>\n <p>It is separate from the From address that a person normally sees when reading the email.<\/p>\n\n <h3>4. SMTP identifies the recipient<\/h3>\n <p>The recipient is specified using:<\/p>\n <p><span class=\"mono\">RCPT TO:<recipient@example.net><\/span><\/p>\n <p>For messages with multiple recipients, the SMTP conversation can contain multiple RCPT TO commands.<\/p>\n\n <h3>5. The message is transferred<\/h3>\n <p>The sending application issues the:<\/p>\n <p><span class=\"mono\">DATA<\/span><\/p>\n <p>command.<\/p>\n <p>It can then transfer the actual email, including its headers, subject, text, HTML and attachments.<\/p>\n <p>Once the SMTP server accepts the message, it can place it in its outgoing mail queue.<\/p>\n\n <h3>6. DNS finds the receiving mail server<\/h3>\n <p>The SMTP server now needs to find out where email for the recipient's domain should be delivered.<\/p>\n <p>If the recipient is:<\/p>\n <p><span class=\"mono\">anna@example.net<\/span><\/p>\n <p>the sending server performs a DNS lookup for the MX records of example.net.<\/p>\n <p>MX stands for Mail Exchange.<\/p>\n <p>These DNS records identify the mail servers responsible for accepting email for the domain.<\/p>\n\n <h3>7. The SMTP server delivers the email<\/h3>\n <p>The sending SMTP server connects to the receiving mail server, normally over port 25.<\/p>\n <p>Another SMTP conversation takes place between the two servers.<\/p>\n <p>The receiving server may examine the:<\/p>\n <ul class=\"card-list\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sending IP address<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sender domain<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SPF authentication<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DKIM signature<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DMARC alignment<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>reverse DNS<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sender reputation<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>message characteristics<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>recipient address<\/li>\n <\/ul>\n <p>The receiving server then decides whether to accept or reject the email.<\/p>\n <p>If accepted, it normally responds with an SMTP success code beginning with:<\/p>\n <p><span class=\"mono\">250<\/span><\/p>\n <p>The SMTP transaction is complete.<\/p>\n <p>The receiving provider then determines where the accepted message should be placed.<\/p>\n <p>That could be the inbox, a category such as Promotions, or the spam folder.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M5 12h14\"><\/path><path d=\"M13 6l6 6-6 6\"><\/path><\/svg>SMTP Delivery in One Line<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>The complete journey can be summarized as:<\/p>\n <div class=\"flow-box\">Application \u2192 SMTP submission \u2192 SMTP server \u2192 DNS\/MX lookup \u2192 recipient SMTP server \u2192 mailbox<\/div>\n <p>That is the basic infrastructure behind internet email.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M17 3l4 4-4 4\"><\/path><path d=\"M3 11V9a4 4 0 014-4h14\"><\/path><path d=\"M7 21l-4-4 4-4\"><\/path><path d=\"M21 13v2a4 4 0 01-4 4H3\"><\/path><\/svg>What Is an SMTP Relay?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>An SMTP relay is a service that accepts email from your application and delivers it to other mail servers on your behalf.<\/p>\n <p>Instead of your webshop, website or application communicating directly with thousands of different mail providers, it sends everything through an SMTP relay.<\/p>\n <p>For example:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">Your webshop \u2192 SMTP relay \u2192 Gmail<\/div>\n <div class=\"flow-box\">Your webshop \u2192 SMTP relay \u2192 Microsoft<\/div>\n <div class=\"flow-box\">Your webshop \u2192 SMTP relay \u2192 Yahoo<\/div>\n <\/div>\n <p>The SMTP provider handles the internet-facing delivery infrastructure.<\/p>\n <p>SMTP relays are commonly used for:<\/p>\n <ul class=\"card-list\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>transactional email<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>password resets<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>login codes<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>invoices and receipts<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>webshop notifications<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SaaS applications<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>CRM systems<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>monitoring systems<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>IoT devices<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>newsletters<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>marketing email<\/li>\n <\/ul>\n\n \n <div class=\"cta-box\">\n <div class=\"cta-content\">\n <div class=\"cta-icon\">\n <svg width=\"20\" height=\"20\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><rect x=\"3\" y=\"5\" width=\"18\" height=\"14\" rx=\"2\"><\/rect><path d=\"M3 7l9 6 9-6\"><\/path><\/svg>\n <\/div>\n <div class=\"cta-text\">\n <h4>Sending newsletters or marketing campaigns through MailWizz?<\/h4>\n <p>Connect MailWizz directly to mySMTP's dedicated SMTP infrastructure for reliable, high-volume delivery.<\/p>\n <\/div>\n <\/div>\n <a class=\"cta-btn\" href=\"https:\/\/mysmtp.com\/solutions\/mailwizz\">MailWizz Integration \u2192<\/a>\n <\/div>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><rect x=\"3\" y=\"3\" width=\"7\" height=\"7\"><\/rect><rect x=\"14\" y=\"3\" width=\"7\" height=\"7\"><\/rect><rect x=\"14\" y=\"14\" width=\"7\" height=\"7\"><\/rect><rect x=\"3\" y=\"14\" width=\"7\" height=\"7\"><\/rect><\/svg>SMTP vs. IMAP vs. POP3<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>SMTP, IMAP and POP3 perform different jobs.<\/p>\n <p>SMTP sends email.<\/p>\n <p>IMAP gives email clients access to messages stored on a mail server and synchronizes them across devices.<\/p>\n <p>POP3 downloads received messages from a mail server.<\/p>\n <p>A simple way to remember the difference is:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">SMTP = send<\/div>\n <div class=\"flow-box\">IMAP \/ POP3 = access received mail<\/div>\n <\/div>\n <p>SMTP therefore delivers a message to the recipient's mail system. IMAP or POP3 may later be used when the recipient accesses that mailbox.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><rect x=\"2\" y=\"7\" width=\"20\" height=\"10\" rx=\"2\"><\/rect><path d=\"M6 12h.01M10 12h.01M14 12h.01\"><\/path><\/svg>What Are the Main SMTP Ports?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>Different SMTP ports are used for different parts of email delivery.<\/p>\n\n <h3>SMTP port 25<\/h3>\n <p>Port 25 is primarily used for SMTP communication between mail servers.<\/p>\n <p>For example:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">mySMTP \u2192 Gmail<\/div>\n <\/div>\n <p>or:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">mySMTP \u2192 Microsoft 365<\/div>\n <\/div>\n <p>Port 25 is frequently restricted by internet providers and hosting companies for end-user connections because of its historical use by spam and compromised systems.<\/p>\n <p>Applications should normally use a submission port instead.<\/p>\n\n <h3>SMTP port 587<\/h3>\n <p>Port 587 is the standard SMTP message submission port and is usually the best default for applications.<\/p>\n <p>It commonly uses STARTTLS, which upgrades the connection to TLS encryption.<\/p>\n\n <h3>SMTP port 465<\/h3>\n <p>Port 465 is used for SMTP submission with implicit TLS.<\/p>\n <p>The TLS-encrypted connection is established immediately.<\/p>\n <p>Port 465 is a valid modern SMTP submission option when supported by your provider.<\/p>\n\n <h3>SMTP port 2525<\/h3>\n <p>Port 2525 is a commonly supported alternative SMTP port.<\/p>\n <p>It is useful when standard SMTP submission ports are restricted by a network or hosting provider.<\/p>\n <p>It is not an official replacement for port 587, but many SMTP providers support it.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M12 2l8 4v6c0 5-3.5 8.5-8 10-4.5-1.5-8-5-8-10V6l8-4z\"><\/path><\/svg>Is SMTP Secure?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>SMTP by itself was created long before today's internet security requirements.<\/p>\n <p>Modern SMTP therefore uses additional security technologies.<\/p>\n\n <h3>TLS<\/h3>\n <p>TLS encrypts the connection between SMTP systems.<\/p>\n <p>It protects SMTP credentials and message content from being transmitted in plain text across the connection.<\/p>\n <p>But encryption is only one part of modern email security.<\/p>\n <p>Three other technologies are especially important:<\/p>\n\n <h3>SPF<\/h3>\n <p>SPF (Sender Policy Framework) identifies which servers or IP addresses are authorized to send email for a domain.<\/p>\n\n <h3>DKIM<\/h3>\n <p>DKIM (DomainKeys Identified Mail) adds a cryptographic signature to an email.<\/p>\n <p>The receiving mail server can verify the signature using a public key published in DNS.<\/p>\n\n <h3>DMARC<\/h3>\n <p>DMARC (Domain-based Message Authentication, Reporting and Conformance) builds on SPF and DKIM and checks their alignment with the domain visible in the From address.<\/p>\n <p>DMARC also allows domain owners to publish a policy for messages that fail authentication.<\/p>\n <p>For professional email sending, SMTP, TLS, SPF, DKIM and DMARC work together.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M21 12V7a2 2 0 00-2-2H5a2 2 0 00-2 2v10a2 2 0 002 2h6\"><\/path><path d=\"M3 7l9 6 9-6\"><\/path><path d=\"M16 16l2 2 4-4\"><\/path><\/svg>Does SMTP Guarantee Inbox Delivery?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>No. SMTP delivery and inbox placement are not the same thing.<\/p>\n <p>This distinction is extremely important.<\/p>\n <p>An SMTP server can successfully deliver an email to Gmail, Microsoft or Yahoo and receive:<\/p>\n <p><span class=\"mono\">250 OK<\/span><\/p>\n <p>That means the receiving mail system accepted the message.<\/p>\n <p>It does not necessarily mean that the email was placed in the inbox.<\/p>\n <p>The receiving provider can still place the message in spam or another folder.<\/p>\n <p>Inbox placement can be influenced by factors such as:<\/p>\n <ul class=\"card-list\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>IP reputation<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>domain reputation<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SPF<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DKIM<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DMARC<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>PTR \/ reverse DNS<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sending history<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sending volume<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>spam complaints<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>bounce rates<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>mailing-list quality<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>recipient engagement<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>message content<\/li>\n <\/ul>\n <p>This is why professional email delivery is about much more than simply having access to an SMTP server.<\/p>\n\n \n <div class=\"featured-cta\">\n <div class=\"featured-left\">\n <span class=\"featured-badge\">\u2726 FEATURED<\/span><span class=\"featured-eyebrow\">TEST.SMTP.AI<\/span><span class=\"featured-live\">Live<\/span>\n <h3>Free Email <span class=\"accent\">Deliverability Test<\/span><\/h3>\n <p>Send a real email to a one-time address and instantly see SPF, DKIM and DMARC results \u2014 before your campaign lands in spam.<\/p>\n <div class=\"featured-checks\">\n <span class=\"featured-check\"><svg width=\"14\" height=\"14\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SPF \u00b7 DKIM \u00b7 DMARC<\/span>\n <span class=\"featured-check\"><svg width=\"14\" height=\"14\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>TLS & PTR check<\/span>\n <span class=\"featured-check\"><svg width=\"14\" height=\"14\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>No signup<\/span>\n <span class=\"featured-check\"><svg width=\"14\" height=\"14\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>Result in seconds<\/span>\n <\/div>\n <div class=\"featured-actions\">\n <a class=\"cta-btn\" href=\"https:\/\/mysmtp.com\/solutions\/emailtest\"><svg width=\"14\" height=\"14\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M22 2L11 13\"><\/path><path d=\"M22 2l-7 20-4-9-9-4 20-7z\"><\/path><\/svg>Start free test<\/a>\n <a class=\"how-it-works\" href=\"https:\/\/mysmtp.com\/solutions\/emailtest\">How it works \u2197<\/a>\n <\/div>\n <\/div>\n <div class=\"mock-browser\">\n <div class=\"mock-browser-bar\">\n <span class=\"mock-dot red\"><\/span><span class=\"mock-dot gray\"><\/span><span class=\"mock-dot green\"><\/span>\n <span class=\"mock-browser-title\">TEST.SMTP.AI<\/span>\n <\/div>\n <div class=\"mock-browser-body\">\n <div class=\"mock-label\">Send to:<\/div>\n <div class=\"mock-input\">sawny.opah.lech@test.smtp.ai<\/div>\n <div class=\"mock-row\"><span>SPF<\/span><span class=\"mock-pass\">\u2713 pass<\/span><\/div>\n <div class=\"mock-row\"><span>DKIM<\/span><span class=\"mock-pass\">\u2713 pass<\/span><\/div>\n <div class=\"mock-row\"><span>DMARC<\/span><span class=\"mock-pass\">\u2713 pass<\/span><\/div>\n <div class=\"mock-row\"><span>TLS<\/span><span class=\"mock-pass\">\u2713 1.3<\/span><\/div>\n <\/div>\n <\/div>\n <\/div>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M12 8v4\"><\/path><path d=\"M12 16h.01\"><\/path><\/svg>What Are SMTP Error Codes?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>SMTP servers communicate using numeric response codes.<\/p>\n <p>These codes tell the sending system whether a command or message was accepted.<\/p>\n <p>The easiest way to understand them is by their first digit.<\/p>\n\n <h3>2xx \u2014 Success<\/h3>\n <p>The SMTP command or message was accepted.<\/p>\n <p>For example:<\/p>\n <p><span class=\"mono\">250 OK<\/span><\/p>\n\n <h3>4xx \u2014 Temporary error<\/h3>\n <p>The server cannot accept the message right now.<\/p>\n <p>Examples include temporary rate limits, server problems and reputation-related throttling.<\/p>\n <p>A properly configured SMTP server will normally queue the message and try again later.<\/p>\n\n <h3>5xx \u2014 Permanent error<\/h3>\n <p>The receiving server has rejected the command or message.<\/p>\n <p>Possible reasons include:<\/p>\n <ul class=\"card-list\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>invalid recipient<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>blocked sender<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>authentication problems<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>policy violations<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>spam filtering<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>configuration errors<\/li>\n <\/ul>\n <p>The text following the SMTP code often provides more information about why the message was rejected.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M8 3L3 8l5 5\"><\/path><path d=\"M16 3l5 5-5 5\"><\/path><path d=\"M10 21l4-18\"><\/path><\/svg>SMTP vs. Email API: What's the Difference?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>Applications can commonly submit email in two ways:<\/p>\n <p>SMTP or an Email API.<\/p>\n <p>SMTP uses the universal SMTP protocol and normally requires:<\/p>\n <ul class=\"card-list single-col\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SMTP hostname<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>username<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>password<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>port<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>encryption settings<\/li>\n <\/ul>\n <p>An email API normally sends the message to the email provider using an HTTPS request.<\/p>\n <p>APIs can provide tighter integration and additional functionality, but SMTP has one major advantage:<\/p>\n <p><strong>It works with an enormous range of existing software and devices.<\/strong><\/p>\n <p>You can often switch SMTP providers simply by changing the SMTP hostname, username and password without changing the application itself.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M12 2l3 7h7l-5.5 4.5L18.5 21 12 16.5 5.5 21l2-7.5L2 9h7z\"><\/path><\/svg>Why Use a Professional SMTP Service?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>Running an SMTP server is possible.<\/p>\n <p>Running one that consistently delivers legitimate business email is considerably more complicated.<\/p>\n <p>A professional SMTP infrastructure needs to manage much more than the SMTP connection itself.<\/p>\n <p>That includes:<\/p>\n <ul class=\"card-list\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>IP addresses<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>IP reputation<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>domain reputation<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sending queues<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>retries<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>bounce processing<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>TLS<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SPF<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DKIM<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DMARC<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>reverse DNS<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>abuse prevention<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>suppression<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>monitoring<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>changing requirements from receiving providers<\/li>\n <\/ul>\n <p>For businesses, the real objective isn't simply:<\/p>\n <p>\"Can we send an email?\"<\/p>\n <p>It is:<\/p>\n <p>\"Will our important email reach the recipient reliably?\"<\/p>\n\n \n <div class=\"cta-box\">\n <div class=\"cta-content\">\n <div class=\"cta-icon\">\n <svg width=\"20\" height=\"20\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><rect x=\"3\" y=\"4\" width=\"18\" height=\"16\" rx=\"2\"><\/rect><path d=\"M3 10h18\"><\/path><path d=\"M7 15h4\"><\/path><\/svg>\n <\/div>\n <div class=\"cta-text\">\n <h4>Ready for a dedicated SMTP infrastructure?<\/h4>\n <p>Compare mySMTP plans for dedicated IPs and transactional or marketing email sending.<\/p>\n <\/div>\n <\/div>\n <a class=\"cta-btn\" href=\"https:\/\/mysmtp.com\/pricing\/marketing-transactional-emails\">View Pricing \u2192<\/a>\n <\/div>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><circle cx=\"12\" cy=\"12\" r=\"3\"><\/circle><path d=\"M12 2v4\"><\/path><path d=\"M12 18v4\"><\/path><path d=\"M4.9 4.9l2.8 2.8\"><\/path><path d=\"M16.3 16.3l2.8 2.8\"><\/path><path d=\"M2 12h4\"><\/path><path d=\"M18 12h4\"><\/path><path d=\"M4.9 19.1l2.8-2.8\"><\/path><path d=\"M16.3 7.7l2.8-2.8\"><\/path><\/svg>What Is a Dedicated IP for SMTP?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>A dedicated SMTP IP is a sending IP address used exclusively by one customer or organization.<\/p>\n <p>With shared SMTP infrastructure, multiple customers send email through the same IP addresses.<\/p>\n <p>With a dedicated IP, the sending organization has greater control over its own sending reputation.<\/p>\n <p>A dedicated IP can be particularly useful for businesses sending regular volumes of transactional or marketing email where reputation, monitoring and predictable delivery are important.<\/p>\n <p>However, a dedicated IP also needs to be managed correctly. Sending patterns, authentication, list quality and complaint rates all influence its reputation.<\/p>\n\n \n <div class=\"cta-box\">\n <div class=\"cta-content\">\n <div class=\"cta-icon\">\n <svg width=\"20\" height=\"20\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M14.7 6.3a1 1 0 000 1.4l1.6 1.6a1 1 0 001.4 0l3.77-3.77a6 6 0 01-7.94 7.94l-6.91 6.91a2.12 2.12 0 01-3-3l6.91-6.91a6 6 0 017.94-7.94l-3.76 3.76z\"><\/path><\/svg>\n <\/div>\n <div class=\"cta-text\">\n <h4>Check your SPF, DKIM, DMARC, MTA-STS and more<\/h4>\n <p>The mySMTP deliverability toolbox is a free suite of email infrastructure tools \u2014 no signup required.<\/p>\n <\/div>\n <\/div>\n <a class=\"cta-btn\" href=\"https:\/\/toolbox.smtp.ai\">Explore the Toolbox \u2192<\/a>\n <\/div>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M9.09 9a3 3 0 015.83 1c0 2-3 2-3 4\"><\/path><path d=\"M12 17h.01\"><\/path><\/svg>Frequently Asked Questions About SMTP<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <div class=\"faq-item\">\n <h3>What is SMTP?<\/h3>\n <p>SMTP stands for Simple Mail Transfer Protocol. It is the standard protocol used to send and transfer email across the internet.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>What is an SMTP server?<\/h3>\n <p>An SMTP server accepts, processes, queues and transfers outgoing email to other mail servers.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>What is an SMTP relay?<\/h3>\n <p>An SMTP relay accepts email from an application or system and delivers it to the recipient's mail server on the sender's behalf.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Is SMTP used for sending or receiving email?<\/h3>\n <p>SMTP is used primarily for sending and transferring email. IMAP and POP3 are commonly used by email clients to access received email.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>What SMTP port should I use?<\/h3>\n <p>For applications, port 587 with STARTTLS is generally the best default. Port 465 with implicit TLS is also a standard option when supported. Port 25 is primarily used for communication between mail servers.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Does SMTP use TCP?<\/h3>\n <p>Yes. SMTP runs over TCP.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Does SMTP use TLS?<\/h3>\n <p>Modern SMTP can use TLS to encrypt connections. Port 587 commonly uses STARTTLS, while port 465 uses implicit TLS.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Do I need SPF, DKIM and DMARC to use SMTP?<\/h3>\n <p>They are separate from SMTP itself, but they are important components of authenticated, professional email delivery and increasingly important for successful delivery to major mailbox providers.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Can SMTP send transactional email?<\/h3>\n <p>Yes. SMTP is widely used for transactional email such as password resets, login codes, invoices, receipts, webshop orders and application notifications.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Can SMTP send marketing email?<\/h3>\n <p>Yes. SMTP can transport both transactional and marketing email. Marketing senders must also manage consent, authentication, list quality, unsubscribes, reputation and applicable legal requirements.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Is SMTP still used in 2026?<\/h3>\n <p>Yes. SMTP remains the core protocol for transferring email between mail servers on the internet.<\/p>\n <p>Newer APIs can change how an application submits a message to an email provider, but SMTP still plays a central role when email moves between mail systems.<\/p>\n <\/div>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M12 2v20\"><\/path><path d=\"M5 9l7-7 7 7\"><\/path><path d=\"M5 15l7 7 7-7\"><\/path><\/svg>SMTP in Short<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>SMTP is the technology that moves email across the internet.<\/p>\n <p>The basic journey is:<\/p>\n <div class=\"flow-box\">Your application \u2192 SMTP server \u2192 recipient's mail server \u2192 mailbox<\/div>\n <p>But modern email delivery involves more than SMTP.<\/p>\n <p>For reliable business email, SMTP works together with TLS, DNS, SPF, DKIM, DMARC, reverse DNS and sender reputation.<\/p>\n <p>That is the difference between simply sending email and operating a professional email delivery infrastructure.<\/p>\n <\/div>\n\n <div class=\"about-box\">\n <h3>About mySMTP<\/h3>\n <p style=\"margin-top:10px;\">mySMTP has provided SMTP services and email deliverability infrastructure since 2007.<\/p>\n <p>We specialize in dedicated SMTP with dedicated sending IP addresses for businesses sending transactional and marketing email.<\/p>\n <p>Our focus is not just moving an email from one server to another. It is providing properly configured SMTP infrastructure with authentication, monitoring and deliverability expertise behind it.<\/p>\n <\/div>\n\n<\/div>\n\n\n<\/body><\/html>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"<p>mySMTP \u00b7 Email Infrastructure Explained SMTP (Simple Mail Transfer Protocol) is the standard protocol used to send email across the internet. It transfers email from an application or email client to an SMTP server and between mail servers until the message reaches the recipient’s email provider. What Is SMTP and How Does It Work? | […]<\/p>\n","protected":false},"author":2,"featured_media":2269,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_oct_exclude_from_cache":false,"footnotes":""},"categories":[3],"tags":[27],"class_list":["post-2267","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-deliverability","tag-deliverability"],"yoast_head":"\n<title>What Is SMTP and How Does It Work? - mySMTP BLOG<\/title>\n<meta name=\"description\" content=\"Learn how SMTP servers send email, how SMTP relay works, which ports to use, and how SPF, DKIM, DMARC and TLS affect delivery.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.mysmtp.blog\/2026\/09\/02\/what-is-smtp-and-how-does-it-work-2\/\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"What Is SMTP and How Does It Work? 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mySMTP \u00b7 Email Infrastructure Explained<\/div>\n \n

SMTP (Simple Mail Transfer Protocol) is the standard protocol used to send email across the internet. It transfers email from an application or email client to an SMTP server and between mail servers until the message reaches the recipient's email provider.<\/div>\n <\/div>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t $\"\"$ \t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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align-items: center; gap: 20px; flex-wrap: wrap; }\n .featured-actions a.how-it-works { font-size: 0.9rem; color: #1a1a1a; font-weight: 500; text-decoration: none; display: inline-flex; align-items: center; gap: 5px; }\n\n .mock-browser {\n width: 300px; flex-shrink: 0; background: white; border-radius: 10px;\n box-shadow: 0 8px 24px rgba(0,0,0,0.1); border: 1px solid #e5e7eb; overflow: hidden;\n }\n .mock-browser-bar { display: flex; align-items: center; gap: 6px; padding: 10px 14px; border-bottom: 1px solid #f0f0f0; }\n .mock-dot { width: 8px; height: 8px; border-radius: 50%; }\n .mock-dot.red { background: #ef4444; } .mock-dot.gray { background: #d1d5db; } .mock-dot.green { background: #22c55e; }\n .mock-browser-title { flex: 1; text-align: center; font-size: 0.65rem; color: #9ca3af; letter-spacing: 0.04em; }\n .mock-browser-body { padding: 16px; }\n .mock-label { font-size: 0.72rem; color: #6b7280; margin-bottom: 6px; }\n .mock-input { font-family: 'Courier New', monospace; font-size: 0.72rem; background: #f9fafb; border: 1px solid #e5e7eb; border-radius: 6px; padding: 8px 10px; margin-bottom: 14px; color: #374151; }\n .mock-row { display: flex; align-items: center; justify-content: space-between; padding: 8px 0; border-top: 1px solid #f3f4f6; font-size: 0.78rem; color: #374151; }\n .mock-row:first-of-type { border-top: none; }\n .mock-pass { color: #16a34a; font-weight: 600; display: inline-flex; align-items: center; gap: 4px; }\n\n \/* \u2500\u2500 FAQ \u2500\u2500 *\/\n .faq-item { border-bottom: 1px solid #e5e7eb; padding: 18px 0; }\n .faq-item:first-child { padding-top: 0; }\n .faq-item:last-child { border-bottom: none; }\n .faq-item h3 { margin: 0 0 6px; }\n .faq-item p { margin: 0; }\n\n \/* \u2500\u2500 ABOUT FOOTER \u2500\u2500 *\/\n .about-box { background: #f9fafb; border-top: 1px solid #e5e7eb; padding: 34px 40px; }\n .about-box p { color: #4b5563; }\n\n @media (max-width: 640px) {\n .header, .section, .about-box, .section-divider { padding-left: 20px; padding-right: 20px; }\n .featured-cta { padding: 24px; }\n .mock-browser { width: 100%; }\n }\n<\/style>\n<\/head>\n<body>\n<div class=\"container\">\n\n \n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M4 4h16v16H4z\"><\/path><path d=\"M4 8h16\"><\/path><\/svg>Overview<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>In simple terms:<\/p>\n <div class=\"flow-box\">Sender \u2192 SMTP server \u2192 recipient's mail server \u2192 inbox<\/div>\n <p>Every time a webshop sends an order confirmation, a website sends a password reset, or a business sends a marketing email, SMTP is part of the delivery process.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M9.09 9a3 3 0 015.83 1c0 2-3 2-3 4\"><\/path><path d=\"M12 17h.01\"><\/path><\/svg>What Does SMTP Stand For?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>SMTP stands for Simple Mail Transfer Protocol.<\/p>\n <p>It is a standard set of rules that allows email systems to communicate with each other when sending and transferring email.<\/p>\n <p>SMTP has existed since the early days of the internet, but it remains the foundation of email delivery today.<\/p>\n <p>Even when an application sends email through an email API, SMTP is normally involved later when the provider transfers the message to the recipient's mail server.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><rect x=\"3\" y=\"4\" width=\"18\" height=\"7\" rx=\"1.5\"><\/rect><rect x=\"3\" y=\"13\" width=\"18\" height=\"7\" rx=\"1.5\"><\/rect><path d=\"M7 7.5h.01M7 16.5h.01\"><\/path><\/svg>What Is an SMTP Server?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>An SMTP server is a mail server that accepts, processes, queues and transfers outgoing email.<\/p>\n <p>Think of it as a specialized delivery system for email.<\/p>\n <p>Your website, application or email program hands the message to an SMTP server. The SMTP server then determines where the message needs to go and attempts to deliver it to the recipient's mail provider.<\/p>\n <p>For example:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">Webshop \u2192 SMTP server \u2192 Gmail \u2192 customer<\/div>\n <\/div>\n <p>or:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">CRM \u2192 SMTP server \u2192 Microsoft 365 \u2192 customer<\/div>\n <\/div>\n <p>The SMTP server handles the technical communication required to move the email between these systems.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M13 2L3 14h7l-1 8 10-12h-7l1-8z\"><\/path><\/svg>How Does SMTP Work?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>When you click Send, several things happen in just a few seconds.<\/p>\n\n <h3>1. The application connects to an SMTP server<\/h3>\n <p>The sending system could be Outlook, WordPress, a webshop, CRM, accounting software, SaaS platform, IoT device or custom application.<\/p>\n <p>It connects to an SMTP server using a hostname such as:<\/p>\n <p><span class=\"mono\">smtp.example.com<\/span><\/p>\n <p>Applications typically connect using SMTP port 587, 465 or sometimes 2525.<\/p>\n\n <h3>2. The SMTP connection starts<\/h3>\n <p>The sending application introduces itself using an SMTP command such as:<\/p>\n <p><span class=\"mono\">EHLO<\/span><\/p>\n <p>The SMTP server responds with the features it supports, which can include authentication and TLS encryption.<\/p>\n <p>If authentication is required, the application proves that it is authorized to use the SMTP server.<\/p>\n\n <h3>3. SMTP identifies the sender<\/h3>\n <p>The sending system provides the envelope sender using:<\/p>\n <p><span class=\"mono\">MAIL FROM:<sender@example.com><\/span><\/p>\n <p>This address is part of the SMTP envelope.<\/p>\n <p>The SMTP envelope contains the routing information mail servers need to deliver the message.<\/p>\n <p>It is separate from the From address that a person normally sees when reading the email.<\/p>\n\n <h3>4. SMTP identifies the recipient<\/h3>\n <p>The recipient is specified using:<\/p>\n <p><span class=\"mono\">RCPT TO:<recipient@example.net><\/span><\/p>\n <p>For messages with multiple recipients, the SMTP conversation can contain multiple RCPT TO commands.<\/p>\n\n <h3>5. The message is transferred<\/h3>\n <p>The sending application issues the:<\/p>\n <p><span class=\"mono\">DATA<\/span><\/p>\n <p>command.<\/p>\n <p>It can then transfer the actual email, including its headers, subject, text, HTML and attachments.<\/p>\n <p>Once the SMTP server accepts the message, it can place it in its outgoing mail queue.<\/p>\n\n <h3>6. DNS finds the receiving mail server<\/h3>\n <p>The SMTP server now needs to find out where email for the recipient's domain should be delivered.<\/p>\n <p>If the recipient is:<\/p>\n <p><span class=\"mono\">anna@example.net<\/span><\/p>\n <p>the sending server performs a DNS lookup for the MX records of example.net.<\/p>\n <p>MX stands for Mail Exchange.<\/p>\n <p>These DNS records identify the mail servers responsible for accepting email for the domain.<\/p>\n\n <h3>7. The SMTP server delivers the email<\/h3>\n <p>The sending SMTP server connects to the receiving mail server, normally over port 25.<\/p>\n <p>Another SMTP conversation takes place between the two servers.<\/p>\n <p>The receiving server may examine the:<\/p>\n <ul class=\"card-list\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sending IP address<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sender domain<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SPF authentication<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DKIM signature<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DMARC alignment<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>reverse DNS<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sender reputation<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>message characteristics<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>recipient address<\/li>\n <\/ul>\n <p>The receiving server then decides whether to accept or reject the email.<\/p>\n <p>If accepted, it normally responds with an SMTP success code beginning with:<\/p>\n <p><span class=\"mono\">250<\/span><\/p>\n <p>The SMTP transaction is complete.<\/p>\n <p>The receiving provider then determines where the accepted message should be placed.<\/p>\n <p>That could be the inbox, a category such as Promotions, or the spam folder.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M5 12h14\"><\/path><path d=\"M13 6l6 6-6 6\"><\/path><\/svg>SMTP Delivery in One Line<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>The complete journey can be summarized as:<\/p>\n <div class=\"flow-box\">Application \u2192 SMTP submission \u2192 SMTP server \u2192 DNS\/MX lookup \u2192 recipient SMTP server \u2192 mailbox<\/div>\n <p>That is the basic infrastructure behind internet email.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M17 3l4 4-4 4\"><\/path><path d=\"M3 11V9a4 4 0 014-4h14\"><\/path><path d=\"M7 21l-4-4 4-4\"><\/path><path d=\"M21 13v2a4 4 0 01-4 4H3\"><\/path><\/svg>What Is an SMTP Relay?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>An SMTP relay is a service that accepts email from your application and delivers it to other mail servers on your behalf.<\/p>\n <p>Instead of your webshop, website or application communicating directly with thousands of different mail providers, it sends everything through an SMTP relay.<\/p>\n <p>For example:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">Your webshop \u2192 SMTP relay \u2192 Gmail<\/div>\n <div class=\"flow-box\">Your webshop \u2192 SMTP relay \u2192 Microsoft<\/div>\n <div class=\"flow-box\">Your webshop \u2192 SMTP relay \u2192 Yahoo<\/div>\n <\/div>\n <p>The SMTP provider handles the internet-facing delivery infrastructure.<\/p>\n <p>SMTP relays are commonly used for:<\/p>\n <ul class=\"card-list\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>transactional email<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>password resets<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>login codes<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>invoices and receipts<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>webshop notifications<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SaaS applications<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>CRM systems<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>monitoring systems<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>IoT devices<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>newsletters<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>marketing email<\/li>\n <\/ul>\n\n \n <div class=\"cta-box\">\n <div class=\"cta-content\">\n <div class=\"cta-icon\">\n <svg width=\"20\" height=\"20\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><rect x=\"3\" y=\"5\" width=\"18\" height=\"14\" rx=\"2\"><\/rect><path d=\"M3 7l9 6 9-6\"><\/path><\/svg>\n <\/div>\n <div class=\"cta-text\">\n <h4>Sending newsletters or marketing campaigns through MailWizz?<\/h4>\n <p>Connect MailWizz directly to mySMTP's dedicated SMTP infrastructure for reliable, high-volume delivery.<\/p>\n <\/div>\n <\/div>\n <a class=\"cta-btn\" href=\"https:\/\/mysmtp.com\/solutions\/mailwizz\">MailWizz Integration \u2192<\/a>\n <\/div>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><rect x=\"3\" y=\"3\" width=\"7\" height=\"7\"><\/rect><rect x=\"14\" y=\"3\" width=\"7\" height=\"7\"><\/rect><rect x=\"14\" y=\"14\" width=\"7\" height=\"7\"><\/rect><rect x=\"3\" y=\"14\" width=\"7\" height=\"7\"><\/rect><\/svg>SMTP vs. IMAP vs. POP3<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>SMTP, IMAP and POP3 perform different jobs.<\/p>\n <p>SMTP sends email.<\/p>\n <p>IMAP gives email clients access to messages stored on a mail server and synchronizes them across devices.<\/p>\n <p>POP3 downloads received messages from a mail server.<\/p>\n <p>A simple way to remember the difference is:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">SMTP = send<\/div>\n <div class=\"flow-box\">IMAP \/ POP3 = access received mail<\/div>\n <\/div>\n <p>SMTP therefore delivers a message to the recipient's mail system. IMAP or POP3 may later be used when the recipient accesses that mailbox.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><rect x=\"2\" y=\"7\" width=\"20\" height=\"10\" rx=\"2\"><\/rect><path d=\"M6 12h.01M10 12h.01M14 12h.01\"><\/path><\/svg>What Are the Main SMTP Ports?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>Different SMTP ports are used for different parts of email delivery.<\/p>\n\n <h3>SMTP port 25<\/h3>\n <p>Port 25 is primarily used for SMTP communication between mail servers.<\/p>\n <p>For example:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">mySMTP \u2192 Gmail<\/div>\n <\/div>\n <p>or:<\/p>\n <div class=\"flow-stack\">\n <div class=\"flow-box\">mySMTP \u2192 Microsoft 365<\/div>\n <\/div>\n <p>Port 25 is frequently restricted by internet providers and hosting companies for end-user connections because of its historical use by spam and compromised systems.<\/p>\n <p>Applications should normally use a submission port instead.<\/p>\n\n <h3>SMTP port 587<\/h3>\n <p>Port 587 is the standard SMTP message submission port and is usually the best default for applications.<\/p>\n <p>It commonly uses STARTTLS, which upgrades the connection to TLS encryption.<\/p>\n\n <h3>SMTP port 465<\/h3>\n <p>Port 465 is used for SMTP submission with implicit TLS.<\/p>\n <p>The TLS-encrypted connection is established immediately.<\/p>\n <p>Port 465 is a valid modern SMTP submission option when supported by your provider.<\/p>\n\n <h3>SMTP port 2525<\/h3>\n <p>Port 2525 is a commonly supported alternative SMTP port.<\/p>\n <p>It is useful when standard SMTP submission ports are restricted by a network or hosting provider.<\/p>\n <p>It is not an official replacement for port 587, but many SMTP providers support it.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M12 2l8 4v6c0 5-3.5 8.5-8 10-4.5-1.5-8-5-8-10V6l8-4z\"><\/path><\/svg>Is SMTP Secure?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>SMTP by itself was created long before today's internet security requirements.<\/p>\n <p>Modern SMTP therefore uses additional security technologies.<\/p>\n\n <h3>TLS<\/h3>\n <p>TLS encrypts the connection between SMTP systems.<\/p>\n <p>It protects SMTP credentials and message content from being transmitted in plain text across the connection.<\/p>\n <p>But encryption is only one part of modern email security.<\/p>\n <p>Three other technologies are especially important:<\/p>\n\n <h3>SPF<\/h3>\n <p>SPF (Sender Policy Framework) identifies which servers or IP addresses are authorized to send email for a domain.<\/p>\n\n <h3>DKIM<\/h3>\n <p>DKIM (DomainKeys Identified Mail) adds a cryptographic signature to an email.<\/p>\n <p>The receiving mail server can verify the signature using a public key published in DNS.<\/p>\n\n <h3>DMARC<\/h3>\n <p>DMARC (Domain-based Message Authentication, Reporting and Conformance) builds on SPF and DKIM and checks their alignment with the domain visible in the From address.<\/p>\n <p>DMARC also allows domain owners to publish a policy for messages that fail authentication.<\/p>\n <p>For professional email sending, SMTP, TLS, SPF, DKIM and DMARC work together.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M21 12V7a2 2 0 00-2-2H5a2 2 0 00-2 2v10a2 2 0 002 2h6\"><\/path><path d=\"M3 7l9 6 9-6\"><\/path><path d=\"M16 16l2 2 4-4\"><\/path><\/svg>Does SMTP Guarantee Inbox Delivery?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>No. SMTP delivery and inbox placement are not the same thing.<\/p>\n <p>This distinction is extremely important.<\/p>\n <p>An SMTP server can successfully deliver an email to Gmail, Microsoft or Yahoo and receive:<\/p>\n <p><span class=\"mono\">250 OK<\/span><\/p>\n <p>That means the receiving mail system accepted the message.<\/p>\n <p>It does not necessarily mean that the email was placed in the inbox.<\/p>\n <p>The receiving provider can still place the message in spam or another folder.<\/p>\n <p>Inbox placement can be influenced by factors such as:<\/p>\n <ul class=\"card-list\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>IP reputation<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>domain reputation<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SPF<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DKIM<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DMARC<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>PTR \/ reverse DNS<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sending history<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sending volume<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>spam complaints<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>bounce rates<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>mailing-list quality<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>recipient engagement<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>message content<\/li>\n <\/ul>\n <p>This is why professional email delivery is about much more than simply having access to an SMTP server.<\/p>\n\n \n <div class=\"featured-cta\">\n <div class=\"featured-left\">\n <span class=\"featured-badge\">\u2726 FEATURED<\/span><span class=\"featured-eyebrow\">TEST.SMTP.AI<\/span><span class=\"featured-live\">Live<\/span>\n <h3>Free Email <span class=\"accent\">Deliverability Test<\/span><\/h3>\n <p>Send a real email to a one-time address and instantly see SPF, DKIM and DMARC results \u2014 before your campaign lands in spam.<\/p>\n <div class=\"featured-checks\">\n <span class=\"featured-check\"><svg width=\"14\" height=\"14\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SPF \u00b7 DKIM \u00b7 DMARC<\/span>\n <span class=\"featured-check\"><svg width=\"14\" height=\"14\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>TLS & PTR check<\/span>\n <span class=\"featured-check\"><svg width=\"14\" height=\"14\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>No signup<\/span>\n <span class=\"featured-check\"><svg width=\"14\" height=\"14\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>Result in seconds<\/span>\n <\/div>\n <div class=\"featured-actions\">\n <a class=\"cta-btn\" href=\"https:\/\/mysmtp.com\/solutions\/emailtest\"><svg width=\"14\" height=\"14\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M22 2L11 13\"><\/path><path d=\"M22 2l-7 20-4-9-9-4 20-7z\"><\/path><\/svg>Start free test<\/a>\n <a class=\"how-it-works\" href=\"https:\/\/mysmtp.com\/solutions\/emailtest\">How it works \u2197<\/a>\n <\/div>\n <\/div>\n <div class=\"mock-browser\">\n <div class=\"mock-browser-bar\">\n <span class=\"mock-dot red\"><\/span><span class=\"mock-dot gray\"><\/span><span class=\"mock-dot green\"><\/span>\n <span class=\"mock-browser-title\">TEST.SMTP.AI<\/span>\n <\/div>\n <div class=\"mock-browser-body\">\n <div class=\"mock-label\">Send to:<\/div>\n <div class=\"mock-input\">sawny.opah.lech@test.smtp.ai<\/div>\n <div class=\"mock-row\"><span>SPF<\/span><span class=\"mock-pass\">\u2713 pass<\/span><\/div>\n <div class=\"mock-row\"><span>DKIM<\/span><span class=\"mock-pass\">\u2713 pass<\/span><\/div>\n <div class=\"mock-row\"><span>DMARC<\/span><span class=\"mock-pass\">\u2713 pass<\/span><\/div>\n <div class=\"mock-row\"><span>TLS<\/span><span class=\"mock-pass\">\u2713 1.3<\/span><\/div>\n <\/div>\n <\/div>\n <\/div>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M12 8v4\"><\/path><path d=\"M12 16h.01\"><\/path><\/svg>What Are SMTP Error Codes?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>SMTP servers communicate using numeric response codes.<\/p>\n <p>These codes tell the sending system whether a command or message was accepted.<\/p>\n <p>The easiest way to understand them is by their first digit.<\/p>\n\n <h3>2xx \u2014 Success<\/h3>\n <p>The SMTP command or message was accepted.<\/p>\n <p>For example:<\/p>\n <p><span class=\"mono\">250 OK<\/span><\/p>\n\n <h3>4xx \u2014 Temporary error<\/h3>\n <p>The server cannot accept the message right now.<\/p>\n <p>Examples include temporary rate limits, server problems and reputation-related throttling.<\/p>\n <p>A properly configured SMTP server will normally queue the message and try again later.<\/p>\n\n <h3>5xx \u2014 Permanent error<\/h3>\n <p>The receiving server has rejected the command or message.<\/p>\n <p>Possible reasons include:<\/p>\n <ul class=\"card-list\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>invalid recipient<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>blocked sender<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>authentication problems<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>policy violations<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>spam filtering<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>configuration errors<\/li>\n <\/ul>\n <p>The text following the SMTP code often provides more information about why the message was rejected.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M8 3L3 8l5 5\"><\/path><path d=\"M16 3l5 5-5 5\"><\/path><path d=\"M10 21l4-18\"><\/path><\/svg>SMTP vs. Email API: What's the Difference?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>Applications can commonly submit email in two ways:<\/p>\n <p>SMTP or an Email API.<\/p>\n <p>SMTP uses the universal SMTP protocol and normally requires:<\/p>\n <ul class=\"card-list single-col\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SMTP hostname<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>username<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>password<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>port<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>encryption settings<\/li>\n <\/ul>\n <p>An email API normally sends the message to the email provider using an HTTPS request.<\/p>\n <p>APIs can provide tighter integration and additional functionality, but SMTP has one major advantage:<\/p>\n <p><strong>It works with an enormous range of existing software and devices.<\/strong><\/p>\n <p>You can often switch SMTP providers simply by changing the SMTP hostname, username and password without changing the application itself.<\/p>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M12 2l3 7h7l-5.5 4.5L18.5 21 12 16.5 5.5 21l2-7.5L2 9h7z\"><\/path><\/svg>Why Use a Professional SMTP Service?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>Running an SMTP server is possible.<\/p>\n <p>Running one that consistently delivers legitimate business email is considerably more complicated.<\/p>\n <p>A professional SMTP infrastructure needs to manage much more than the SMTP connection itself.<\/p>\n <p>That includes:<\/p>\n <ul class=\"card-list\">\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>IP addresses<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>IP reputation<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>domain reputation<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>sending queues<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>retries<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>bounce processing<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>TLS<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>SPF<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DKIM<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>DMARC<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>reverse DNS<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>abuse prevention<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>suppression<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>monitoring<\/li>\n <li><svg width=\"13\" height=\"13\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"3\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M8 12l2.5 2.5L16 9\"><\/path><\/svg>changing requirements from receiving providers<\/li>\n <\/ul>\n <p>For businesses, the real objective isn't simply:<\/p>\n <p>\"Can we send an email?\"<\/p>\n <p>It is:<\/p>\n <p>\"Will our important email reach the recipient reliably?\"<\/p>\n\n \n <div class=\"cta-box\">\n <div class=\"cta-content\">\n <div class=\"cta-icon\">\n <svg width=\"20\" height=\"20\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><rect x=\"3\" y=\"4\" width=\"18\" height=\"16\" rx=\"2\"><\/rect><path d=\"M3 10h18\"><\/path><path d=\"M7 15h4\"><\/path><\/svg>\n <\/div>\n <div class=\"cta-text\">\n <h4>Ready for a dedicated SMTP infrastructure?<\/h4>\n <p>Compare mySMTP plans for dedicated IPs and transactional or marketing email sending.<\/p>\n <\/div>\n <\/div>\n <a class=\"cta-btn\" href=\"https:\/\/mysmtp.com\/pricing\/marketing-transactional-emails\">View Pricing \u2192<\/a>\n <\/div>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><circle cx=\"12\" cy=\"12\" r=\"3\"><\/circle><path d=\"M12 2v4\"><\/path><path d=\"M12 18v4\"><\/path><path d=\"M4.9 4.9l2.8 2.8\"><\/path><path d=\"M16.3 16.3l2.8 2.8\"><\/path><path d=\"M2 12h4\"><\/path><path d=\"M18 12h4\"><\/path><path d=\"M4.9 19.1l2.8-2.8\"><\/path><path d=\"M16.3 7.7l2.8-2.8\"><\/path><\/svg>What Is a Dedicated IP for SMTP?<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>A dedicated SMTP IP is a sending IP address used exclusively by one customer or organization.<\/p>\n <p>With shared SMTP infrastructure, multiple customers send email through the same IP addresses.<\/p>\n <p>With a dedicated IP, the sending organization has greater control over its own sending reputation.<\/p>\n <p>A dedicated IP can be particularly useful for businesses sending regular volumes of transactional or marketing email where reputation, monitoring and predictable delivery are important.<\/p>\n <p>However, a dedicated IP also needs to be managed correctly. Sending patterns, authentication, list quality and complaint rates all influence its reputation.<\/p>\n\n \n <div class=\"cta-box\">\n <div class=\"cta-content\">\n <div class=\"cta-icon\">\n <svg width=\"20\" height=\"20\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M14.7 6.3a1 1 0 000 1.4l1.6 1.6a1 1 0 001.4 0l3.77-3.77a6 6 0 01-7.94 7.94l-6.91 6.91a2.12 2.12 0 01-3-3l6.91-6.91a6 6 0 017.94-7.94l-3.76 3.76z\"><\/path><\/svg>\n <\/div>\n <div class=\"cta-text\">\n <h4>Check your SPF, DKIM, DMARC, MTA-STS and more<\/h4>\n <p>The mySMTP deliverability toolbox is a free suite of email infrastructure tools \u2014 no signup required.<\/p>\n <\/div>\n <\/div>\n <a class=\"cta-btn\" href=\"https:\/\/toolbox.smtp.ai\">Explore the Toolbox \u2192<\/a>\n <\/div>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><circle cx=\"12\" cy=\"12\" r=\"10\"><\/circle><path d=\"M9.09 9a3 3 0 015.83 1c0 2-3 2-3 4\"><\/path><path d=\"M12 17h.01\"><\/path><\/svg>Frequently Asked Questions About SMTP<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <div class=\"faq-item\">\n <h3>What is SMTP?<\/h3>\n <p>SMTP stands for Simple Mail Transfer Protocol. It is the standard protocol used to send and transfer email across the internet.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>What is an SMTP server?<\/h3>\n <p>An SMTP server accepts, processes, queues and transfers outgoing email to other mail servers.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>What is an SMTP relay?<\/h3>\n <p>An SMTP relay accepts email from an application or system and delivers it to the recipient's mail server on the sender's behalf.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Is SMTP used for sending or receiving email?<\/h3>\n <p>SMTP is used primarily for sending and transferring email. IMAP and POP3 are commonly used by email clients to access received email.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>What SMTP port should I use?<\/h3>\n <p>For applications, port 587 with STARTTLS is generally the best default. Port 465 with implicit TLS is also a standard option when supported. Port 25 is primarily used for communication between mail servers.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Does SMTP use TCP?<\/h3>\n <p>Yes. SMTP runs over TCP.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Does SMTP use TLS?<\/h3>\n <p>Modern SMTP can use TLS to encrypt connections. Port 587 commonly uses STARTTLS, while port 465 uses implicit TLS.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Do I need SPF, DKIM and DMARC to use SMTP?<\/h3>\n <p>They are separate from SMTP itself, but they are important components of authenticated, professional email delivery and increasingly important for successful delivery to major mailbox providers.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Can SMTP send transactional email?<\/h3>\n <p>Yes. SMTP is widely used for transactional email such as password resets, login codes, invoices, receipts, webshop orders and application notifications.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Can SMTP send marketing email?<\/h3>\n <p>Yes. SMTP can transport both transactional and marketing email. Marketing senders must also manage consent, authentication, list quality, unsubscribes, reputation and applicable legal requirements.<\/p>\n <\/div>\n <div class=\"faq-item\">\n <h3>Is SMTP still used in 2026?<\/h3>\n <p>Yes. SMTP remains the core protocol for transferring email between mail servers on the internet.<\/p>\n <p>Newer APIs can change how an application submits a message to an email provider, but SMTP still plays a central role when email moves between mail systems.<\/p>\n <\/div>\n <\/div>\n\n \n <div class=\"section-divider\">\n <div class=\"section-divider-label\"><svg width=\"15\" height=\"15\" viewBox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\"><path d=\"M12 2v20\"><\/path><path d=\"M5 9l7-7 7 7\"><\/path><path d=\"M5 15l7 7 7-7\"><\/path><\/svg>SMTP in Short<\/div>\n <div class=\"section-divider-line\"><\/div>\n <\/div>\n <div class=\"section\">\n <p>SMTP is the technology that moves email across the internet.<\/p>\n <p>The basic journey is:<\/p>\n <div class=\"flow-box\">Your application \u2192 SMTP server \u2192 recipient's mail server \u2192 mailbox<\/div>\n <p>But modern email delivery involves more than SMTP.<\/p>\n <p>For reliable business email, SMTP works together with TLS, DNS, SPF, DKIM, DMARC, reverse DNS and sender reputation.<\/p>\n <p>That is the difference between simply sending email and operating a professional email delivery infrastructure.<\/p>\n <\/div>\n\n <div class=\"about-box\">\n <h3>About mySMTP<\/h3>\n <p style=\"margin-top:10px;\">mySMTP has provided SMTP services and email deliverability infrastructure since 2007.<\/p>\n <p>We specialize in dedicated SMTP with dedicated sending IP addresses for businesses sending transactional and marketing email.<\/p>\n <p>Our focus is not just moving an email from one server to another. It is providing properly configured SMTP infrastructure with authentication, monitoring and deliverability expertise behind it.<\/p>\n <\/div>\n\n<\/div>\n\n\n<\/body><\/html>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"<p>mySMTP \u00b7 Email Infrastructure Explained SMTP (Simple Mail Transfer Protocol) is the standard protocol used to send email across the internet. It transfers email from an application or email client to an SMTP server and between mail servers until the message reaches the recipient’s email provider. What Is SMTP and How Does It Work? | […]<\/p>\n","protected":false},"author":2,"featured_media":2269,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_oct_exclude_from_cache":false,"footnotes":""},"categories":[3],"tags":[27],"class_list":["post-2267","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-deliverability","tag-deliverability"],"yoast_head":"\n<title>What Is SMTP and How Does It Work? - mySMTP BLOG<\/title>\n<meta name=\"description\" content=\"Learn how SMTP servers send email, how SMTP relay works, which ports to use, and how SPF, DKIM, DMARC and TLS affect delivery.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.mysmtp.blog\/2026\/09\/02\/what-is-smtp-and-how-does-it-work-2\/\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"What Is SMTP and How Does It Work? 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