Enhanced SMTP status code · RFC 3463 · IANA

5.1.1 Recipient mailbox does not exist

The part left of the @ in the recipient's address is not valid: the mailbox does not exist on the server that answered.

Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1). The registry says this code is only useful as a permanent failure.
Subject: X.1 · Address
About the originator or destination address: syntax or validity. The sender can usually correct it and retry.
Associated basic codes (IANA registry)
451550
Source
RFC 3463 · IANA

How 5.1.1 appears at providers

The exact text from the provider's documentation, as you see it in the bounce, and what it means.

550 5.1.1The email account that you tried to reach does not exist. Please double-check the recipient's email address for typos or unnecessary spaces.

The recipient's address does not exist. Check for typos and stray spaces.

5.1.1Bad destination mailbox address

The recipient's address does not exist or is misspelled (or the address cached by Outlook is stale).

Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026