5.1.4: recipient address is ambiguous
The recipient address matches more than one mailbox on the receiving system, so the server could not decide which person should get the message.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.1.4
- 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
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
5.1.4 Destination mailbox address ambiguous
What it means
Some mail systems map addresses to people loosely, for example by surname or partial name. 5.1.4 says that mapping found several candidates and the server will not guess. RFC 3463 describes it as the result of such a heuristic lookup, and RFC 5321 shows the matching reply, 553 for an ambiguous user, in its examples. It is rare on the public internet, where an address maps to exactly one mailbox, and more likely with older directory-based systems or gateways. RFC 3463 ties it to no class: 5.1.4 means the message was returned, 4.1.4 that the server will accept a retry. A more precise address solves it.
Common causes
- A short or partial address, such as a first name or surname, that several people on the system share.
- A directory or gateway that resolves addresses by matching names.
How to fix it
- 01Ask the recipient for their full, exact address.
- 02If the bounce lists possible matches, pick the right one and resend.
- 03Recipient admins: give each mailbox a unique address and switch off name guessing for inbound mail.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.1.1Mailbox 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.
- 553Mailbox name not allowed.The server refused the address because the mailbox name is not allowed there, most often because it is written incorrectly.
All 105 standard codesEvery code in the wikiPaste your own bounce