SMTP enhanced status code 5.1.0

What the registry says

In practice

X.1.0 is used by servers that know an address was the problem but do not classify it further. In practice it most often comes back for a recipient the server will not accept for reasons it does not disclose, such as an address-level block or a recipient that failed a directory lookup. Look at which address it names. If it is the recipient and other mail to that domain is delivered, the address itself is the likely problem, and a mailbox check will usually say whether it exists.

What the sender should do

Check both the recipient and your own envelope sender address for typos and stray characters. If the recipient is the problem, a mailbox check before sending shows whether it exists.

How providers use X.1.0

Quoted from each provider’s own error reference.

• Microsoft 365 5.1.0

  Sender denied

Common questions

What does 5.1.0 mean?

X.1.0 says something about an address in the message caused the failure, without saying which address or what is wrong with it. Most servers use it for a recipient they could not accept for an unspecified address reason.

What is the difference between 5.1.0 and 4.1.0?

They share the subject and detail digits, so they report the same condition, "Other address status". 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 5.1.0?

Check both the recipient and your own envelope sender address for typos and stray characters. If the recipient is the problem, a mailbox check before sending shows whether it exists.

Would an email verifier have caught 5.1.0?

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.

Full guides for this code

• 550 5.1.0 — address rejected, other address status