SMTP Error 511 - Bad destination mailbox address — a misread of 5.1.1, not a real code. | Warmup Inbox

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.

Most rejections come back to sender reputation.

Blocklists, auth failures, and policy blocks are what warmup prevents. Warm up your domain so your mail is trusted before you hit send.