Email Deliverability

SMTP Authentication Failed

SMTP authentication failure occurs when an email client or application cannot authenticate with the mail server. This typically results in 535 (authentication failed) or 550 (relay access denied) errors. Causes range from incorrect credentials and disabled app passwords to server-side authentication mechanism mismatches.

Symptoms

  • Error 535 5.7.8 "Authentication credentials invalid"
  • Error 550 "Relay access denied"
  • Email client cannot send messages but can receive
  • Applications using SMTP suddenly stop sending emails

Common Causes

  • Incorrect username or password in SMTP client configuration
  • Two-factor authentication enabled without app-specific password
  • SMTP authentication method mismatch (PLAIN vs LOGIN vs CRAM-MD5)
  • Account locked due to too many failed login attempts
  • SMTP server requiring TLS/SSL but client using unencrypted connection

Step-by-Step Troubleshooting

  1. 1

    Verify SMTP settings

    Check SMTP host, port, and encryption settings match your provider requirements.

    Use HTTP Headers →
  2. 2

    Test SMTP TLS

    Verify TLS connectivity to the mail server on the expected port.

    Use SMTP TLS Test →
  3. 3

    Check MX records

    Confirm MX records point to the correct mail server.

    Use MX Record Lookup →
  4. 4

    Generate app password

    If using 2FA, create a dedicated app password for SMTP access in your email provider settings.

  5. 5

    Verify SPF includes SMTP server

    Ensure your SPF record authorizes the SMTP server IP to send on your behalf.

    Use SPF Checker →

Recommended Diagnostic Tools

SMTP TLS TestMX Record LookupSPF Checker

Monitor SMTP Health

Track SMTP server availability and authentication status.

Related Problems

Frequently Asked Questions