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](/content/smtp-error-codes/x-1-1/index.html)

## Related codes

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

## 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.