SMTP error 553 means the receiving server has rejected the mailbox name in the envelope. Either the address is malformed (typo, illegal characters, missing TLD), or the server refuses to relay because the From or recipient domain isn’t one it accepts. It’s a hard bounce: no automatic retry will save it.
Read the message « 553 mailbox name not allowed », « 553 5.1.8 sender address rejected », « 553 relaying denied »: each variant points at a different cause.
The three usual causes
| Variant | Likely cause | Fix |
|---|---|---|
| Mailbox name not allowed | Typo or illegal character in the address | Correct the address (no spaces, no extra dots, valid domain). |
| Sender address rejected | Your From domain is unreachable or unauthenticated | Use a real domain you own and authenticate it. |
| Relaying denied | The destination server doesn’t relay for your sender | Send through Mailpro’s servers, not a third-party relay you don’t control. |
How to prevent recurring 553 errors
Use double opt-in to keep typos out of your list, validate new contacts at signup, and authenticate your sending domain with SPF and DKIM. A misconfigured From domain will keep producing 553 even if every recipient address is valid.
Action on a 553 It’s a hard bounce. Remove the address from the campaign list and fix the cause before sending again.
Authenticate and clean before resending
Configure SPF and DKIM for your sending domain, and read the broader guide on common SMTP errors.