Permanent failure — do not retry
SMTP 551 5.1.6
mailbox has moved
SMTP 551 5.1.6 means the recipient's mailbox has moved and the server is not forwarding mail to it. RFC 3463 defines X.1.6 as "destination mailbox has moved, no forwarding address". The person may still be reachable, but not at this address, so stop sending to it.
How it appears
551 5.1.6 Recipient address has changed and is no longer valid
551 User not local; please try <user@example.org>
What causes it
- The person left the organisation and the mailbox was retired.
- The company migrated to a new domain and dropped the old addresses.
- An alias was removed during a mail system change.
How to fix it
In order — try the first one first.
- Remove the old address from your list.
- If the bounce names a new address, update the contact only if you have the person's consent to email the new one.
- If many addresses at one company return 5.1.6, check whether the company changed domain.
Common questions
- What does SMTP 551 5.1.6 mean?
- SMTP 551 5.1.6 means the recipient's mailbox has moved and the server is not forwarding mail to it. RFC 3463 defines X.1.6 as "destination mailbox has moved, no forwarding address". The person may still be reachable, but not at this address, so stop sending to it.
- Should I retry a 551 5.1.6 error?
- No. 551 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 551 5.1.6?
- Remove the old address from your list.
Other 551 replies
Last reviewed 2026-09-11