Soft bounce

Soft bounces are about timing, not the address

A soft bounce is a temporary failure, signalled by a 4xx SMTP code, or a message the receiving server accepted and then reported as undeliverable for a transient reason. The usual causes:

• The mailbox is full (reported as 452 4.2.2 by some servers and as a permanent error by others, so classification is inconsistent).
• The server is down or overloaded.
• Greylisting: the server deliberately deferred an unknown sender.
• The message is too large, or the recipient's server is rate-limiting you.

How retries should work

A sending system retries a soft bounce on a schedule for a while, typically over a few days, and only then reports it as failed. If your tool gives up after one attempt, it is turning recoverable mail into losses.

When a soft bounce stops being soft

The same address soft-bouncing repeatedly over weeks is effectively dead: an abandoned mailbox that never empties, or a domain that no longer routes mail. Most tools convert it to a hard bounce after a few consecutive failures. Treat it the same: suppress it.

A cluster of soft bounces from one provider at the same moment, with healthy delivery elsewhere, points at throttling of your domain, which is a reputation problem and not a list problem. Compare with a hard bounce.