SMTP enhanced status code 5.1.6

What the registry says

In practice

X.1.6 is rarer than 5.1.1, because most servers simply report an unknown user once an account is removed. Where it appears, it is a clear signal that the person has gone, and some systems include a note such as mailbox moved or no longer valid. Treat it exactly like 5.1.1 for list hygiene. If you send to business contacts, a batch of X.1.6 bounces from one company can also mean that company changed domain.

What the sender should do

Treat it as a hard bounce and remove the address. If the contact matters, find their new address through another channel.

Common questions

What does 5.1.6 mean?

X.1.6 means the mailbox used to exist but no longer accepts mail and the server has no forwarding address to offer. It is how some systems report a person who has left an organisation or closed an account.

What is the difference between 5.1.6 and 4.1.6?

They share the subject and detail digits, so they report the same condition, "Destination mailbox has moved, No forwarding address". 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 5.1.6?

Treat it as a hard bounce and remove the address. If the contact matters, find their new address through another channel.

Would an email verifier have caught 5.1.6?

Yes, verification catches this. A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.

Full guides for this code

• 551 5.1.6 — mailbox has moved