550 5.1.1 Recipient address rejected: User unknown
Sample bounce line
550 5.1.1 <[email protected]>: Recipient address rejected: User unknown
What it means
RFC 3463 defines X.1.1 as Bad destination mailbox address: the mailbox in the address does not exist, and for Internet mail names the part left of the @ is invalid. The class digit 5 marks a permanent failure that is not likely to be resolved by resending the message in its current form. The 'Recipient address rejected: User unknown' wording is Postfix's.
Why it happens
- The part of the address before the @ is misspelled or the mailbox never existed or was removed
- Postfix: the recipient is in a local domain but is not listed in $local_recipient_maps (LOCAL_RECIPIENT_README: rejected with 'User unknown in local recipient table')
How to fix it
- Sender: check the spelling of the address and confirm it with the recipient by another channel
- Postfix admin: make sure local_recipient_maps lists the databases that really hold your valid recipients (LOCAL_RECIPIENT_README gives examples such as $alias_maps for UNIX recipients and $virtual_mailbox_maps for virtual delivery), then add any missing recipient
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