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Deliverability Letter

550 5.1.1

550 5.1.1 User Unknown

Invalid Recipient

SFR confirmed the recipient does not exist. Treat as a hard bounce.

User unknown bounces count toward SFR_IN_104. Remove the address immediately to protect reputation.

Severity

ERROR

Provider

SFR

Last Reviewed

2025-06-01

How to Fix 550 5.1.1

Follow these steps to resolve this SFR SMTP error and restore normal delivery.

  • ✓Delete the address after the first bounce and avoid further attempts.
  • ✓Validate signups with double opt-in for French subscribers.
  • ✓Synchronise suppression lists between ESPs and transactional systems.
  • ✓Alert stakeholders when VIP contacts bounce so they can verify the mailbox.

Helpful Resources

More SFR SMTP Codes

Keep these related troubleshooting guides handy while addressing 550 5.1.1.

421 4.7.0 – 421 4.7.0 Too Many Errors
WARNING
450 4.7.1 – SFR_IN_101 Spam Complaints
ERROR
450 4.7.1 – SFR_IN_102 Virus or Malware Detected
CRITICAL
450 4.7.1 – SFR_IN_103 Too Many SMTP Errors
WARNING

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