3-digit reply code Permanent

SMTP error 511

Bad destination mailbox address — a misread of 5.1.1, not a real code.

Updated Jul 1, 2026

The short answer

Bad destination mailbox address — a misread of 5.1.1, not a real code. Common cause: The recipient mailbox does not exist. The real bounce is 550 5.1.1.

Quick fix

Correct the recipient address and remove dead addresses from your list.

5 - Permanent. The server will not try again. Fix the cause before you resend.

How to fix it

Correct the recipient address and remove dead addresses from your list.

Paired code

A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:

X.1.1 Bad destination mailbox address

Related codes

513 Non-standard label: bad address syntax, or a relay/auth failure. 517 Non-standard label: bad sender attribute, or a policy block. 522 Mailbox full — a non-standard label for 552 / 5.2.2. 533 Non-standard label, usually a misread of 5.3.3. 510 Invalid recipient address — a vendor label, not a real code. 512 Domain not found — a vendor label for enhanced status 5.1.2.
All SMTP error codes
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