SMTP enhanced status code 5.1.1

What the registry says

The basic codes it travels with

• 451 Requested action aborted: local error in processing
• 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)

In practice

Every major provider sends 550 5.1.1 for a non-existent user: Gmail says the account does not exist, Microsoft reports a bad destination mailbox address, and Postfix servers say user unknown. A handful in a large send is normal list decay, since addresses stop working when people change jobs or close accounts. A high share in one send means the list was old, scraped or bought. Mailbox providers count these against the sending domain, which is why suppressing them at once matters more than the single lost message.

What the sender should do

Remove the address. Look for an obvious typo first, then suppress it so no future campaign retries it. If many 5.1.1 bounces arrive from one send, verify the rest of that list before the next one.

How providers use X.1.1

Quoted from each provider’s own error reference.

• Gmail 550 5.1.1

  The email account that you tried to reach does not exist. Please double-check the recipient's email address for typos or unnecessary spaces. For more information, go to Fix bounced or rejected emails.

• Microsoft 365 5.1.1

  Bad destination mailbox address

Common questions

What does 5.1.1 mean?

X.1.1 means the mailbox does not exist: the part of the address before the @ is not a user on that domain. As 5.1.1 it is the classic hard bounce, and it is the code an email verifier exists to catch before you send.

What is the difference between 5.1.1 and 4.1.1?

They share the subject and detail digits, so they report the same condition, "Bad destination mailbox 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 5.1.1?

Remove the address. Look for an obvious typo first, then suppress it so no future campaign retries it. If many 5.1.1 bounces arrive from one send, verify the rest of that list before the next one.

Would an email verifier have caught 5.1.1?

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

• 550 5.1.1 — mailbox does not exist