What is a soft bounce?
Also called: temporary failure, 4xx bounce, deferral
A soft bounce is a temporary delivery failure: a full mailbox, a server that is down, a message that is too large, a rate limit or greylisting. It returns a 4xx SMTP code and the sending server retries automatically, usually over hours or days, before giving up.
Soft bounces are normal in small numbers. A server that is briefly unreachable, or a recipient who has not cleared their inbox, produces a soft bounce that resolves itself on the next attempt.
They matter for verification because a temporary failure during a mailbox probe is not evidence about the address. A verifier that treats it as a verdict either invents an invalid result for a working address or caches a wrong answer. The correct report is that the address is unconfirmed.
Persistent soft bounces to the same address over weeks behave like hard bounces and the address should be suppressed. A sudden rise in soft bounces across one provider usually means you are being rate limited there, which is a reputation signal rather than a list problem.
Example
452 4.2.2 The email account that you tried to reach is over quota. The first digit 4 means try again later.How SimpleVerifier handles it
A temporary refusal during the probe is reported as risky · Server would not confirm and is never cached as a verdict, so the next check asks the server again.
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Sources
By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms