SMTP Error Codes

Common error codes and troubleshooting guide for SMTP.

← Back to SMTP Test Page

SMTP Error Codes: Causes and Solutions

SMTP error codes are numerical indications of email transaction statuses that help diagnose when emails fail to send or receive. These three-digit codes communicate between email servers to indicate success or failure.

Understanding SMTP Codes

Each code consists of three digits with specific meanings:

  • First digit: Indicates response class (positive, negative, or incomplete)
  • Second digit: Provides category details
  • Third digit: Offers specific error information

Code Categories

2xx - Success Codes

Indicate successful email operations:

  • 250: Request completed successfully
  • 251: Non-local recipient accepted; server assumes responsibility

4xx - Temporary Error Codes

Signal issues that may resolve automatically; retry later:

  • 421: Service unavailable
  • 450: Mailbox temporarily unavailable (possible server overload)
  • 451: Processing error; temporary failure

5xx - Permanent Error Codes

Require corrective action before resending:

  • 530: Authentication required
  • 550: Requested action not taken; mailbox unavailable
  • 551: User not local; forward address unavailable
  • 552: Storage exceeded; email cannot be accepted
  • 553: Mailbox name not allowed; formatting issue
  • 554: Transaction failed (usually authentication issues)

Common Causes

  • Misconfigured server settings
  • Incorrect DNS records
  • Authentication errors
  • Full or restricted mailboxes
  • Invalid email addresses

Troubleshooting Steps

  1. Review error logs for specific codes
  2. Verify DNS and server settings
  3. Check network connectivity
  4. Confirm authentication credentials

Prevention Best Practices

  • Conduct regular server maintenance
  • Validate email addresses before sending
  • Keep software and configurations current
  • Monitor server performance and loads