550 5.1.1 is a permanent SMTP reply meaning recipient address does not exist. The recipient's mail server is telling you "no such user here." The local part (the bit before the @) does not match any mailbox, alias, or distribution list on that domain.
What 550 5.1.1 means
The recipient's mail server is telling you "no such user here." The local part (the bit before the @) does not match any mailbox, alias, or distribution list on that domain.
Common causes
The address is mistyped or has a typo
Most commonA single wrong character (jonh@ vs john@) produces this bounce. The most common cause by far.
The mailbox was deleted or the person left
CommonEmployees leave and mailboxes get removed; old addresses on your list then bounce 5.1.1.
You're mailing a stale or purchased list
CommonHigh 5.1.1 rates across many recipients signal an old or low-quality list — which also hurts your sender reputation.
How to fix 550 5.1.1
- 1
Verify the exact address
Confirm the spelling with the recipient or check it against a known-good source. For a single contact, this is usually all it takes.
- 2
Remove bouncing addresses from your list
Permanently suppress any address that returns 5.1.1 — continuing to mail it damages your reputation and can land you on a blacklist.
- 3
Use list validation before big sends
For marketing lists, run email verification to catch non-existent mailboxes before they bounce and drag down your deliverability.
Confirm your domain is set up correctly
Most delivery failures — permanent or temporary — trace back to authentication or reputation. Run a free EmailHealth check to confirm SPF, DKIM, DMARC, blacklist status, and TLS are all healthy for your sending domain. If you send in volume, free daily monitoring will alert you before a small issue becomes a flood of bounces.