User unknown
A delivery failure because the recipient address does not exist at the destination domain, reported with reply code 550 and enhanced status code 5.1.1.
What it means
RFC 3463 defines X.1.1 as a bad destination mailbox address: the mailbox named in the address does not exist. The receiving server usually says so in reply to RCPT TO, with text such as 550 5.1.1 User unknown, and providers use their own wording: Yahoo says the user does not have an account, and Microsoft lists causes from a mistyped address to one that does not exist in the destination system.
It is the classic hard bounce: the address will not start working on retry, so it should be suppressed at once. Receivers notice senders who hit many addresses that do not exist: Xfinity rejects messages sent to too many invalid recipients, and Outlook.com blocks IPs that probe for valid addresses, which it calls namespace mining.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 3463, section 3.2: Address Statusrfc-editor.org
- Yahoo Sender Hub: SMTP error codessenders.yahooinc.com
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
- Xfinity Support: Email error codesxfinity.com
- Outlook.com Postmaster: Troubleshootingsubstrate.office.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes