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A soft bounce is a temporary email delivery failure signaled by a 4xx SMTP response. Unlike a hard bounce, the address itself is usually valid; something transient prevented delivery. Sending systems typically queue the message and retry over a period of hours or days before giving up.
Common causes include a recipient mailbox that is full, a receiving server that is temporarily down or overloaded, a message that exceeds size limits, or greylisting, where the server deliberately defers a first attempt to test whether the sender retries like a legitimate mail system would.
While a single soft bounce is harmless, a pattern matters. An address that soft bounces on every send for weeks is effectively dead and should be treated like a hard bounce and suppressed. Repeatedly hammering a server that keeps deferring you can also look abusive and harm your reputation.
For senders, the practical rule is to let your platform handle automatic retries but set a limit: after a defined number of consecutive soft bounces, move the address to a suppression list. This keeps your active list clean while still giving genuinely temporary problems time to resolve.