Email bounce: what the code means before you retry anything

An email bounce is a delivery failure returned by the receiving server. The distinction that governs every decision afterwards is between a hard bounce — a permanent failure such as an address that does not exist — and a soft bounce, a temporary condition such as a full mailbox or a server that is briefly unavailable.

Hard bounces must be suppressed at once and never retried. Continuing to send to addresses that do not exist is one of the clearest signals that a sender is not maintaining its list, and receivers use exactly that signal when deciding where the rest of your mail goes.

Reading the email bounce code

• A 5xx response is permanent: no such user, domain not found, mailbox disabled, blocked as spam.
• A 4xx response is temporary: mailbox full, greylisting, rate limiting, temporary DNS failure.
• Enhanced status codes give more detail than the class alone and are worth logging in full.
• The text after the code often names the actual reason, including a policy URL when a receiver has blocked you.
• A message accepted and then discarded silently is not a bounce at all — that is a filtering outcome, visible only in engagement or in DMARC reports.

The last case is the one that misleads teams: a clean bounce rate does not mean mail is arriving. Acceptance means the server took the message, not that it reached an inbox.

What to do with each

1. Hard bounce: suppress the address permanently, and remove it from every list, not just the one that sent.
2. Soft bounce: retry on a decaying schedule, and convert to a suppression after several consecutive failures.
3. Block or policy rejection: read the reason and the URL — this is about your sending, not the recipient, and retrying makes it worse.
4. Investigate a sudden rise in bounces before sending the next campaign; a spike usually means a list problem or an authentication change.
5. Log the codes, not just the counts, so patterns by domain are visible.

Where a bounce rate above a few percent appears on a list that used to be clean, the cause is usually an imported list, a long gap since the last send, or an authentication change that receivers are now enforcing. Sending more, or sending faster, resolves none of these. Rising bounces are an email deliverability signal, not only a list problem.

Preventing the ones you can

Most hard bounces are preventable: confirm the address at the point of capture, use double opt-in for marketing lists, re-verify lists that have not been mailed for months, and never buy a list — the bounces alone will damage the domain, quite apart from the legal position under privacy law.

Authentication matters here too. Mail that fails SPF or DKIM alignment is more likely to be rejected outright, and those rejections arrive as bounces that look like recipient problems but are yours. Checking the DMARC reports when bounces rise usually explains more than reading the bounce text.

Ettex Mail keeps the delivery result against each message, so a bounce is a state of the record with its code retained rather than a notification somebody deleted. Suppression is the part that has to be honoured everywhere the address appears — and no tool substitutes for the discipline of not sending to people who never asked.