A soft bounce corresponds to a temporary SMTP failure, typically a 4xx response, meaning the receiving server rejected the message for now but the address itself is not known to be invalid. Common causes include a full mailbox, a greylisting delay, or a temporary server outage.
Soft bounces are retried with backoff rather than suppressed outright, but a small number of providers repeatedly soft-bounce mail as a disguised block; if an address soft-bounces on every attempt over an extended window, it's treated as effectively undeliverable and suppressed.
Confusing a persistent soft bounce with a genuinely temporary one is a common source of reputation damage from senders who keep retrying long past the point it stopped being useful.