Glossary · Deliverability
A soft bounce is a temporary delivery failure, like a full mailbox or a busy server, signalled by a 4xx SMTP reply; the sender retries later.
Temporary errors cover full mailboxes, servers that are down or rate-limiting, and greylisting (deliberately deferring unknown senders once). The sending server queues the message and retries, usually for a few days, before giving up.
One soft bounce means little. An address that soft-bounces on every send for weeks is effectively dead and should be treated like a hard bounce. A sudden wave of deferrals from one provider can mean it's throttling you over reputation.
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