SMTP error X.1.1
Bad destination mailbox address
Updated Jul 1, 2026
SMTP 511 is not a real reply code. It is almost always the enhanced status 5.1.1 ("bad destination mailbox address") with the dots removed, which travels with a 550 or 553 bounce meaning the recipient address does not exist. Correct the typo'd or invalid address and remove dead addresses from your list.
Check for typos and verify the recipient address
What it means
There is no 511 reply code in RFC 5321. In the 5xx range the address-related codes are 550, 551, 552, 553, and 554. "511" is a misreading of the enhanced status 5.1.1, which appears in a bounce right next to the real reply code, like "550 5.1.1 Recipient address rejected: User unknown". Strip the dots from 5.1.1 and it reads as 511. Per RFC 3463, 5.1.1 means the mailbox to the left of the @ does not exist. The leading 5 makes it permanent: resending the same message unchanged will not work.
Common causes
The part before the @ is wrong. A misspelling is the most common cause.
The employee left, or the alias was removed.
The address parses fine but points at no real mailbox.
How to fix it
- Read the full bounce line
Capture the real reply code (550 vs 553) and the text after 5.1.1, which states the actual reason ("User unknown", "No such user").
- Correct the local part
Fix the part before the @: check for typos, stale usernames, or a wrong address.
- Confirm the mailbox exists
Ask the recipient for their current address, or send a test to a known-good address on the same domain to rule out a server-wide problem.
- Remove the address from active sends
Because 5.1.1 is permanent, suppress or delete it rather than retrying. Mailing addresses that hard-bounce hurts your reputation.
- Validate at intake
Use double opt-in and address validation to catch invalid recipients before they bounce. A spike in 5.1.1 across many recipients usually means bad list data, not individual typos.
Common questions
Is 511 the same as 550?
They appear together. A real bounce reads "550 5.1.1 User unknown": 550 is the reply code and 5.1.1 is the enhanced code. "511" is just 5.1.1 without the dots; there is no standalone 511 reply code.
Why do I get 5.1.1 for addresses that look correct?
The address may parse fine but point at no real mailbox, or the list data is stale. Confirm the mailbox exists with the recipient.
Should I retry an email that returned 5.1.1?
No. It is permanent. Correct or remove the address instead.