SMTP enhanced status code 2.1.5

What the registry says

The basic codes it travels with

• 250 Requested mail action okay, completed

In practice

You see 2.1.5 in two places: in the RCPT TO reply during an SMTP conversation, such as 250 2.1.5 Recipient OK, and in positive delivery status notifications. A verifier reads this reply to tell that a mailbox exists. On its own it is not proof of a real inbox, because catch-all domains answer 2.1.5 for every address, so a verifier also tests whether the domain accepts made-up addresses before trusting it.

What the sender should do

Nothing to fix. If you see 2.1.5 in a log, the recipient address was accepted at that stage of the conversation.

Common questions

What does 2.1.5 mean?

X.1.5 is a success code: the destination mailbox address is valid. You see it as 2.1.5 in positive delivery reports or in reply to RCPT TO, and it is the answer a mailbox check is looking for.

What should I do about 2.1.5?

Nothing to fix. If you see 2.1.5 in a log, the recipient address was accepted at that stage of the conversation.

Would an email verifier have caught 2.1.5?

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.