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SMTP Bounce Code 5.1.1: User Unknown

SMTP bounce code 5.1.1 indicates that the recipient's email address does not exist on the destination mail server. This is one of the most common hard bounce errors you'll encounter in email delivery, and it means the email address is invalid or has been deleted.

What Does 5.1.1 Mean?

The enhanced status code 5.1.1 follows the SMTP Enhanced Status Code format:

  • 5 = Permanent failure (hard bounce)
  • 1 = Addressing status (related to the recipient address)
  • 1 = Bad destination mailbox address (user unknown)

When you receive a 5.1.1 bounce, it means the receiving mail server has confirmed that the email address you're trying to reach doesn't exist in their system. This is a definitive response—the address is invalid and should be removed from your mailing list.

Bounce Type

  • Type: Hard bounce (permanent failure)
  • Category: Mailbox
  • Action Required: Remove the email address from your mailing list immediately

Common Causes

  1. Typo in Email Address: The recipient's email address was entered incorrectly (e.g., [email protected] instead of [email protected])
  2. Deleted Account: The email account has been deleted or deactivated by the recipient or their email provider
  3. Never Existed: The email address was never valid in the first place
  4. Domain Mismatch: The domain portion of the email address doesn't match the actual domain (e.g., sending to [email protected] when the domain is actually example.org)
  5. Account Suspension: The recipient's email account has been suspended or disabled by the email provider

How to Resolve

For Email Marketers

  1. Remove from Mailing List: Immediately remove the email address from your active mailing list to maintain good sender reputation
  2. Verify Email Address: Double-check the email address for typos or errors before adding it to your list
  3. Use Double Opt-in: Implement double opt-in to ensure email addresses are valid before sending campaigns
  4. Clean Your List Regularly: Periodically clean your email list to remove invalid addresses
  5. Monitor Bounce Rates: Keep your hard bounce rate below 2% to maintain good deliverability

For Developers

  1. Validate Email Format: Use proper email validation before storing addresses in your database
  2. Implement Bounce Handling: Set up automated bounce processing to remove hard bounces from your database
  3. Use Email Verification Services: Consider using email verification APIs to check addresses before sending
  4. Track Bounce Codes: Log bounce codes in your system to identify patterns and improve data quality

Related Bounce Codes

Examples

Example Bounce Message

550 5.1.1 User unknown
The email account that you tried to reach does not exist.

Example Enhanced Status Code

550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in virtual mailbox table

Common Email Provider Responses

  • Gmail: "The email account that you tried to reach does not exist"
  • Outlook/Hotmail: "550 5.1.1 User unknown"
  • Yahoo: "550 Invalid recipient"
  • Custom Domains: Varies by mail server configuration

Best Practices

  1. Never Retry: Don't attempt to resend emails to addresses that bounce with 5.1.1—they will continue to fail
  2. Maintain List Hygiene: Regularly clean your email list to remove invalid addresses
  3. Monitor Metrics: Track your hard bounce rate and aim to keep it below 2%
  4. Use Suppression Lists: Maintain a suppression list of bounced addresses to prevent future sends
  5. Implement Real-time Validation: Validate email addresses at the point of collection to prevent invalid addresses from entering your system