5.1.1: mailbox does not exist
The recipient's domain accepts mail, but the mailbox named before the @ does not exist there, so the message was refused and will not be delivered.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.1.1
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
5.1.1 Bad destination mailbox address
What it means
5.1.1 is the standard way to say the address is unknown: the receiving server handles the domain, looked up the local part (the text before the @) and found no such mailbox. RFC 3463 reserves it for permanent failures, and it usually arrives with reply code 550 and an immediate bounce. It is the textbook hard bounce. A 4.1.1 is occasionally sent by servers that could not verify the address at that moment and want the sender to try again; that is a temporary state, so let the retry happen. Sending again to an address that returned 5.1.1 does not help, and repeated attempts at unknown users count against a sender's reputation with mailbox providers.
Common causes
- A typo in the part before the @, such as a missing letter or swapped name order.
- The person left the organisation and their mailbox was deleted.
- An old or bought list holding addresses that were closed long ago.
- A form or import that accepted a made-up address.
How to fix it
- 01Compare the address in the bounce, character by character, with what the recipient gave you.
- 02Remove the address from your list, or mark it as a hard bounce in your sending tool, so it is not mailed again.
- 03If the address should exist, reach the recipient another way and ask them to check it with their IT team.
- 04Confirm new sign-ups (double opt-in) so unknown addresses never reach your list.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 550Mailbox unavailable.The receiving server permanently refused the message for this recipient: the mailbox does not exist, cannot be reached, or the server rejected the message by policy.
- 5.1.6Mailbox has moved.The address used to be valid, but the mailbox no longer takes mail and the server has no new address to forward the message to.
- 5.1.2Recipient domain cannot receive mail.The part of the address after the @ names a domain that does not exist or cannot accept mail, so the message cannot be delivered there.
- 556 5.1.10Recipient domain has a null MX.The recipient's domain publishes a null MX record, its formal statement that it never receives email, so the message was refused at once without any delivery attempt.
5.1.1 at other providers
- 550 5.1.1Recipient address does not exist.There is no Gmail or Google Workspace account at the address you wrote to, so Gmail rejected the message permanently.
- 550 5.1.1Recipient address not found.The address does not exist in the destination system, is mistyped, or is cached in Outlook with details that are no longer valid.
- 550 5.1.1Recipient address does not exist.Fastmail could find no user with the address you wrote to. Check the spelling; if it is right, the account may be inactive or closed.
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce