SMTP 550 5.1.1: user unknown – causes & fix

daniele@mikweb: ~/errors — less smtp-550-5-1-1-user-unknown.md

What the message looks like

550 5.1.1 <anna@example.com>: Recipient address rejected: User unknown in virtual mailbox table

Typo in the address, or the mailbox was deleted.
Address missing from virtual_mailbox_maps, or alias missing from virtual_alias_maps.
Hash file not rebuilt with postmap after editing.
Domain listed in mydestination and in virtual_mailbox_domains – Postfix then looks for local Unix users (“User unknown in local recipient table”).
After a migration the MX still points to the old server, which no longer has the mailboxes.

Show Postfix domain classes and maps
postconf mydestination virtual_mailbox_domains virtual_mailbox_maps virtual_alias_maps
Check whether the address is a known mailbox
postmap -q anna@example.com $(postconf -h virtual_mailbox_maps)
Check whether an alias exists for the address
postmap -q anna@example.com $(postconf -h virtual_alias_maps)
Find rejected recipients in the log
journalctl -u postfix --since '1 hour ago' | grep 'User unknown'

Have the sender check the address – often it is just a typo.
Create the mailbox or alias; for hash maps run postmap /etc/postfix/virtual and postfix reload.
Put every domain in exactly one class: local domains in mydestination, virtual ones only in virtual_mailbox_domains.
After migrations check the MX record and TTL so mail does not land on the old server.
Do not add a catch-all as a workaround: it attracts spam and hides real errors.

No. The code is only about the recipient. Your reputation is not affected unless you send in bulk to invalid addresses.

Usually the SQL query in the map returns nothing, e.g. a typo in the query or the wrong column. Test with postmap -q directly against the map file.

daniele@mikweb: ~/blog — ls

daniele@mikweb: ~/errors — ls