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Common SMTP Errors & Solutions

⚠️ Common SMTP Connection Issues

Here are the most frequent connection problems and what they typically mean:

1. Connection Timed Out

Error Example:
Connection timed out or Could not connect to SMTP host

Cause:
The client could not reach the SMTP server. This is often due to:

  • Network issues
  • Firewall rules blocking port 25 or 587
  • SMTP server being down or unreachable

Fix:

  • Check your internet/network connection.
  • Ensure the SMTP server address is correct.
  • Confirm that the relevant SMTP ports are open.
  • Try telnet to test connectivity:
    telnet smtp.example.com 587

2. Authentication Failed

Error Example:
535 Authentication failed or SMTP Error: Could not authenticate

Cause:
The server rejected the login credentials.

Fix:

  • Double-check the username and password.
  • Make sure the SMTP server requires authentication (some internal relays do not).
  • Ensure the account is active and not locked.

3. TLS/SSL Errors

Error Example:
SSL: CERTIFICATE_VERIFY_FAILED or Handshake failed

Cause:
Issues with establishing a secure connection:

  • Outdated SSL/TLS protocols
  • Invalid or expired server certificates
  • Client doesn't trust the server certificate authority

Fix:

  • Make sure your client supports modern TLS versions (TLS 1.2+).
  • Verify the SMTP server’s certificate.
  • Update system CA trust store or disable certificate validation (for testing only).

4. Blocked by Firewall or ISP

Error Example:
No specific error, but connection silently fails or times out.

Cause:
Many ISPs block outbound traffic on port 25 to reduce spam.

Fix:

  • Try using ports 587  (with STARTTLS).
  • Contact your ISP to unblock port 25.
  • Use a relay service like mySMTP that supports alternate ports.

5. Too Many Connections / Throttling

Error Example:
421 Too many concurrent SMTP connections or 421 Service not available

Cause:
The server is limiting how many connections a single client can make.

Fix:

  • Implement connection pooling or reuse connections.
  • Reduce sending rate.
  • Work with your ESP to increase connection limits if needed.

6. DNS Issues

Error Example:
Temporary failure in name resolution

Cause:
SMTP server hostname can't be resolved.

Fix:

  • Check that the hostname is correct.
  • Ensure your server has working DNS settings.
  • Try using the SMTP server’s IP address as a test.

🛠️ General Troubleshooting Steps

  1. Ping or traceroute the SMTP server.
  2. Use telnet or openssl to manually test the connection.
  3. Check logs from your email client or sending application.
  4. Verify server status from your email service provider.
  5. Temporarily disable antivirus/firewall to rule them out.
  6. Try from another network to isolate the problem.

✅ Tips for Reliable SMTP Connections

  • Use modern encryption (TLS 1.2+).
  • Don’t hardcode IPs—use hostnames that can be updated.
  • Monitor connection rates and timeouts.
  • Use connection retries with backoff strategies.
  • Keep your client libraries updated.