Email bounce-back messages explained (what the codes mean)
A bounce-back is the mail server telling you why delivery failed. 5xx = permanent (bad address — suppress it); 4xx = temporary (retry). Here's how to read the codes and act on them.
Short answer: an email bounce-back is the receiving mail server returning your message with a reason code. 5xx codes are permanent failures (e.g. 550 "no such user" — suppress that address immediately and never retry); 4xx codes are temporary (e.g. 421/450 "mailbox busy / greylisted" — safe to retry over a day or two). Reading these correctly is the difference between a clean sender reputation and a blacklisted domain.
How to read bounce codes
- 550 / 5.1.1 — mailbox doesn't exist → permanent, suppress.
- 552 / 5.2.2 — mailbox full → usually permanent after retries.
- 554 / 5.7.x — rejected for policy/spam/reputation → investigate sender config.
- 421 / 4.x.x — temporary (greylisting, rate limit) → retry with backoff.
What to do with each
Suppress hard (5xx) bounces on the first hit — they count heavily against reputation. Retry soft (4xx) bounces a few times, then treat persistent ones as hard. Keep total bounce rate under 2.5%.
Autocloz classifies every bounce, auto-suppresses hard bounces, retries soft ones, and auto-pauses a mailbox if bounces spike — so a bad list can't quietly burn your domain.
> Start free — automatic bounce classification + suppression, built in.