What it looks like in your logs
551 5.1.6 User not local; please try <user@new.example.com>What does 551 mean?
RFC 5321 defines 551 as "user not local; please try" followed by a forward path. The receiving server knows the recipient is not hosted there and refuses to relay the message on your behalf. It is the refusing counterpart of 251, where a server accepts the message and forwards it itself. Your server will not retry it, because the answer is permanent for that address.
You rarely see 551 today. Most servers either accept and forward mail or reject unknown recipients outright with 550. When 551 does appear, it is usually after a mailbox move or a domain migration, or when MX records still point at a server that no longer hosts the mailbox. Some servers include the new address in the reply text, which is the most useful part of the bounce.
The enhanced code that often accompanies 551 is 5.1.6, which RFC 3463 defines as destination mailbox has moved with no forwarding address. Large consumer providers such as Gmail and Outlook.com almost never emit 551. If you do see it, read the full reply before touching the contact record, because the suggested forward path is the one piece of information that can save the address. Without one, the safest move is to suppress it and ask the person directly for a current address.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 551 | ✓ this code |
Common causes
- The recipient mailbox moved to another server or domain.
- The domain's MX records still point to an old server that no longer hosts the mailbox.
- An organization is partway through a mail migration.
- The server you connected to refuses to relay mail for domains it does not host.
How to fix 551
- Read the bounce text for a suggested forward path and update the contact record if one is given.
- Check the recipient domain's MX records with the MX lookup to see whether they point to the right server.
- If no new address is offered, suppress the old one and ask the contact for a current address through another channel.
- Do not keep resending to the old address, since 551 is a permanent failure.
551 questions
What does SMTP error 551 "user not local" mean?
It means the server you delivered to does not host that recipient and will not pass the message along. The reply sometimes includes the address or server you should use instead. It is a permanent failure for the address you tried, so update or suppress it.
What is the difference between 251 and 551?
Both say the recipient is not local. With 251, the server accepts the message and forwards it to the right place. With 551, the server refuses and leaves it to you to resend to the correct address. 251 is a success code; 551 is a hard bounce.
Should I remove an address that returned 551?
Suppress the original address. If the bounce names a new address and the contact opted in, you can update the record. Otherwise treat it like any other invalid address and follow the steps in our list hygiene guide. Keep a note of why it was suppressed so nobody reimports it by accident.