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: a bounce-back is the receiving mail server handing your message back with a reason code. 5xx codes are permanent failures — e.g. 550 "no such user," which you suppress immediately and never retry. 4xx codes are temporary — e.g. 421/450 "mailbox busy / greylisted," safe to retry over a day or two. Read them right and you keep a clean sender reputation; read them wrong and you blacklist your own 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 — every one counts heavily against your reputation. Retry soft (4xx) bounces a few times, then treat the ones that keep failing as hard. Hold total bounce rate under 2.5%.
Autocloz classifies every bounce, auto-suppresses hard bounces, retries soft ones, and auto-pauses a mailbox the moment bounces spike — so a bad list can't quietly torch your domain.
> Start free — automatic bounce classification + suppression, built in.