smtp bounce code
550 5.1.1User unknown
The mailbox does not exist. This is the one code that genuinely means the address is dead, and the only one worth suppressing on.
What the server is telling you
The receiving server looked up the local part and found nothing. It is not a filter, a policy or a temporary condition: there is no mailbox at that address. Google phrases it as "the email account that you tried to reach does not exist", Postfix as "Recipient address rejected: User unknown in virtual mailbox table".
Next steps
- →Suppress the address permanently. Retrying will never work.
- →If it used to work, someone left the company or the mailbox was deleted.
- →Check for a typo first — a single wrong character produces this exact code.
Commonly returned by: Gmail, Outlook, Postfix, Exim, almost everything
Find these before you send, not after
Paste a list and see which addresses sit on domains that accept no mail, which are one character from a real inbox, and which are on catch-all domains where nothing can tell you anything. Free, no signup, nothing stored.
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