Introduction

A 550 SMTP error is a permanent failure indicating that the recipient's mail server has rejected the email address. This can happen because the address does not exist, the mailbox is disabled, the domain has no valid MX record, or the receiving server's anti-spam policies are blocking the message. Unlike temporary errors (4xx codes), 550 errors will not resolve with retry attempts.

Symptoms

  • Email bounces immediately with 550 5.1.1 User unknown or 550 Recipient address rejected
  • Mail server logs show RCPT TO rejected with specific reason codes
  • Multiple recipients bounce with the same error pattern
  • Bounce message includes Diagnostic-Code: smtp; 550
  • Error message: 550 5.1.1 <user@example.com>: Recipient address rejected: User unknown in virtual mailbox table

Common Causes

  • Typo in the recipient email address
  • Recipient account was deleted or disabled
  • Domain MX records point to a server that does not recognize the recipient
  • Recipient mail server greylisting or anti-spam rejecting the sender
  • Email address format mismatch (e.g., user@subdomain.example.com vs user@example.com)

Step-by-Step Fix

  1. 1.Verify the recipient address is correct: Check for typos or format issues.
  2. 2.```bash
  3. 3.# Check the address format
  4. 4.echo "user@exmaple.com" | grep -E "^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$"
  5. 5.# Returns nothing for typo
  6. 6.`
  7. 7.Check the recipient domain's MX records: Verify the domain accepts email.
  8. 8.```bash
  9. 9.dig MX example.com +short
  10. 10.# Should return one or more mail exchangers
  11. 11.# If no MX records, check for A record fallback
  12. 12.dig A example.com +short
  13. 13.`
  14. 14.Test the recipient address via SMTP: Verify the server accepts the address.
  15. 15.```bash
  16. 16.telmail example.com user
  17. 17.# Or manually:
  18. 18.telmail mail.example.com 25
  19. 19.EHLO example.com
  20. 20.MAIL FROM:<sender@example.com>
  21. 21.RCPT TO:<user@example.com>
  22. 22.# 250 = accepted, 550 = rejected
  23. 23.`
  24. 24.Contact the recipient through an alternative channel: Confirm the address is valid.
  25. 25.`
  26. 26.# Reach out via phone, messaging, or alternative email
  27. 27.# Verify the correct email address
  28. 28.# Ask if their mailbox is active and accepting email
  29. 29.`
  30. 30.Remove the invalid address from mailing lists: Prevent future bounces.
  31. 31.```bash
  32. 32.# Remove from mailing list
  33. 33.# Update CRM or contact database
  34. 34.# Mark as bounced in email marketing platform
  35. 35.`

Prevention

  • Implement email address validation at the point of entry (web forms, imports)
  • Use email verification services to validate addresses before sending
  • Monitor bounce rates and automatically remove addresses that hard bounce
  • Maintain a suppression list of permanently bounced addresses
  • Double-check recipient addresses before sending critical emails
  • Implement email confirmation workflows for new subscriber signups