Generic SMTP
550 5.1.1 User unknown — the recipient address does not exist
550 5.1.1
The receiving system could not find a valid destination mailbox for the address. RFC 3463 defines 5.1.1 as a bad destination mailbox address: the address does not exist, even though the surrounding domain may be valid. This is a permanent failure for that exact address unless the recipient or administrator corrects the mailbox configuration.
How to fix it
- 1
Check the complete address
Look for a typo before and after the @ sign, an outdated surname, a copied punctuation mark, or an incorrect alias.
- 2
Verify through another channel
Ask the recipient or their administrator to confirm the current address and whether the mailbox or alias still exists.
- 3
Check directory sync and aliases if you manage the domain
Confirm the user is licensed or enabled for mail, the address is attached to the intended mailbox, and directory changes have reached every receiving gateway.
- 4
Suppress the unchanged address
Do not keep retrying a verified 5.1.1 response. Remove or suppress it until someone supplies a corrected destination.
Meaning current as of 2026-07-15, cross-checked against the provider's own documentation. Providers do occasionally redefine codes — if this doesn't match what you're seeing, the source link is the authoritative reference.