What is a soft bounce?
A soft bounce is a temporary delivery failure — a full mailbox, a server that is down, a message that is too large, or greylisting. It returns a 4xx SMTP code and the sending server will retry automatically, usually over several hours or days, before giving up.
Soft bounces are normal and mostly harmless in small numbers. A mail 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 soft bounce during a mailbox probe is not evidence about the address. If a verifier treats a temporary failure as a verdict, it either invents an invalid result for a working address or, worse, caches that wrong answer. A correct verifier retries and, failing that, reports the address as unconfirmed.
Persistent soft bounces to the same address over weeks are effectively hard bounces and should be removed from the list.
Last reviewed 2026-09-11