SMTP enhanced status code 5.1.4

What the registry says

In practice

Ambiguous-address replies are uncommon on the public internet because most systems require an exact match on the local part. They appear on some corporate and university systems that try to map a name like j.smith to a directory entry, and find more than one person. Retrying the same address will keep failing. If the recipient matters, the fix is to get the precise address from them, or use the one in their email signature.

What the sender should do

Ask the recipient for their full, exact address and use that instead. Do not retry the ambiguous form.

Common questions

What does 5.1.4 mean?

X.1.4 means the address you used matched more than one mailbox on the destination system, so the server will not guess which one you meant. It is rare and mostly seen on systems that map short or partial names to users.

What is the difference between 5.1.4 and 4.1.4?

They share the subject and detail digits, so they report the same condition, "Destination mailbox address ambiguous". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.

What should I do about 5.1.4?

Ask the recipient for their full, exact address and use that instead. Do not retry the ambiguous form.

Would an email verifier have caught 5.1.4?

Partly. The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.