Generic SMTP delivery response
Understand SMTP 5.1.1: Recipient mailbox does not exist
The receiving system could not find the mailbox part of the destination address.
A representative response is:
550 5.1.1 Recipient mailbox does not exist.
This is permanent for the submitted address. Correct or provision the recipient before sending again.
What this response means
Standard 5.1.1 Bad destination mailbox address means the receiving system could not find the recipient mailbox. It is not a sender-authentication failure.
Fix it
- Confirm the complete address with the recipient through another channel.
- Type it into a new message instead of resending the failed item or selecting a stale suggestion.
- If you manage the receiving domain, verify that the mailbox, alias, group, or mail-enabled object exists.
- Check recent renames, migrations, and directory synchronization if the address used to work.
- Suppress a permanently bouncing address from automated lists until it is corrected.
SPF, DKIM, DMARC, and sender DNS changes cannot create a missing recipient.
Who can fix it
The sender can correct a typo; the receiving-mail administrator can create or restore the intended recipient.
Related troubleshooting
Use an MX or routing guide only if every address at the destination domain fails with routing evidence.
Frequently asked questions
What does SMTP 5.1.1: Recipient mailbox does not exist mean?
The mailbox named to the left of the recipient @ sign does not exist in the destination mail system.
Can MailFixIt prove this entire bounce from public DNS?
No. The current checker can evaluate selected public DNS configuration, but it cannot recreate recipient state, credentials, reputation, message content, or a historical provider SMTP decision.
Related email diagnostics
Primary sources: IANA SMTP Enhanced Status Codes.