Permanent failure — do not retry
SMTP 551
user not local
SMTP 551 means the recipient is not hosted on this server. RFC 5321 defines it as "user not local; please try <forward-path>": the server knows about the person but will not forward for them, and may name a new address. It is permanent for this address.
How it appears
551 User not local; please try <user@example.org>
551 5.1.6 Recipient address has changed and is no longer valid
What causes it
- The person has moved to a different address or domain and the server is pointing you there.
- The server will not relay mail for users it does not host.
- An old alias was retired after a migration.
How to fix it
In order — try the first one first.
- Read the text after the code. If it names a new address, update the contact (with their consent) rather than retrying the old one.
- If no new address is given, remove the old one.
- If you see 551 across a whole domain, the organisation has probably migrated mail providers. Check its MX records.
Common questions
- What does SMTP 551 mean?
- SMTP 551 means the recipient is not hosted on this server. RFC 5321 defines it as "user not local; please try <forward-path>": the server knows about the person but will not forward for them, and may name a new address. It is permanent for this address.
- Should I retry a 551 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?
- Read the text after the code. If it names a new address, update the contact (with their consent) rather than retrying the old one.
Specific 551 replies
Last reviewed 2026-09-11