Soft bounce
Industry term, not a standards term, for a temporary delivery failure where the message is valid but a transient condition prevented delivery. It corresponds to RFC 3463's persistent transient failure class, 4.X.X.
In plain terms
A temporary failure. Keep the address and try again.
Why it matters for deliverability
Soft bounces are noise individually and signal in aggregate. A single one means nothing; a rising soft-bounce rate at one provider usually means throttling, which is a reputation warning before it becomes a blocking problem.
What to do about it
- Retry on a backoff schedule instead of suppressing.
- Set a threshold — repeated soft bounces to the same address over several sends indicate an abandoned mailbox.
- Chart soft-bounce rate per provider, not just overall.
Source
RFC 3463 — Enhanced Mail System Status Codes (basis for the 4.X.X transient class)
“A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay”
Checked 2026-08-29
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