Soft bounce

Also called: Temporary bounce, Transient failure, Deferral.

RFC 5321 reply codes starting with 4 mean transient failure: the server can't accept the message now, but might later. Typical causes are a full mailbox, a server that's down or overloaded, rate limiting, and greylisting, where a server deliberately defers unknown senders on the first attempt.

Sending servers retry soft bounces automatically for a while. If the address keeps failing, treat it like a hard bounce and remove it. Yahoo advises monitoring both hard and soft bounces, and Google recommends reducing your sending volume if mail starts being deferred, until the error rate drops.

In email verification, a temporary reply is not an answer. A verifier that grades a 4xx reply as invalid throws away good addresses; the right result is unknown, with a retry later.

A sudden rise in soft bounces across many domains usually means providers are throttling you, which is a reputation signal. Slow down rather than pushing through.

Example

A message to a salon's address returns 452 4.2.2 Mailbox full. The sender retries over the next day, then suppresses the address after repeated failures across two campaigns.