Bounce rate
The share of a send that fails to deliver. It is only meaningful when split into permanent failures (RFC 3463 class 5.X.X) and transient ones (class 4.X.X), because the two say different things about your list and your infrastructure.
In plain terms
How much of your send did not arrive — and, more usefully, why not.
Why it matters for deliverability
A single blended bounce-rate figure hides the distinction that matters. A high permanent rate is a list-quality problem you caused before you pressed send; a high transient rate is usually a throughput or reputation problem happening right now.
What to do about it
- Report permanent and transient bounce rates as separate numbers.
- Classify from the enhanced status code, not from bounce-message wording.
- Verify lists before sending, since permanent bounces cost reputation the moment they happen.
Source
RFC 3463 — Enhanced Mail System Status Codes (the permanent/transient classification)
“5.XXX.XXX Permanent Failure — A permanent failure is one which is not likely to be resolved by resending the message in the current form.”
Checked 2026-08-29
Go deeper
Definitions reviewed 2026-08-29. Spotted something wrong? Tell us.
See how your own mail is handled
Run a real message through MailTester and read the authentication results, spam score and delivery signals described on this page against your own domain.