What is a soft bounce?
A soft bounce is a temporary email delivery failure where the recipient address is valid but the message can't be delivered right now — for example because the mailbox is full or the server is temporarily unavailable. The send may succeed when retried.
Common causes of soft bounces
Soft bounces are returned as 4.x.x SMTP responses and signal a transient problem rather than a bad address. Typical causes include a recipient mailbox that's over quota, a mail server that's down or overloaded, a message that exceeds the recipient's size limit, or greylisting that defers a first-time sender.
Because the address itself is valid, mail servers and ESPs automatically retry soft-bounced messages over a period of hours or days before giving up.
How soft and hard bounces are treated differently
A hard bounce is permanent and the address should be suppressed immediately. A soft bounce is temporary, so ESPs keep retrying and only escalate to a suppression after a configured number of consecutive failures.
Monitoring matters: an address that soft-bounces repeatedly over weeks is effectively undeliverable and should be treated like a hard bounce, because continuing to mail it wastes reputation with no chance of delivery.
Reducing soft bounces
You can't control a recipient's full mailbox, but you can avoid self-inflicted soft bounces: keep message sizes reasonable, warm up new sending IPs gradually so providers don't defer you, and authenticate your mail with SPF, DKIM, and DMARC so servers don't greylist you as suspicious.
Verification still helps indirectly — a clean list keeps your reputation high, which means providers are less likely to defer or throttle your messages in the first place.
In practice
You email a valid subscriber whose mailbox is full. The server returns 452 mailbox full — a soft bounce. Your ESP retries over the next 24 hours; once the recipient clears space, the message is delivered. No suppression is needed unless the soft bounce persists across many sends.
Frequently asked questions
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