What Is Soft Bounce?
A soft bounce is a temporary email delivery failure — the recipient’s mailbox is full, the server is down, or the message is too large. Unlike hard bounces, soft bounces may succeed on retry, but persistent soft bounces to the same address should eventually be treated as undeliverable.
Soft bounces return temporary SMTP error codes (4xx). Sending servers automatically retry over a period (often 24–72 hours) before giving up. Common triggers include full mailboxes, greylisting, temporary server outages, and oversized messages.
While a single soft bounce is harmless, a pattern matters. If an address soft-bounces repeatedly across several campaigns, treat it like a hard bounce and suppress it — the mailbox may be abandoned, and continued attempts waste reputation.
Some soft bounces are actually greylisting, a spam-defense tactic where the server temporarily defers unknown senders and accepts the retry. Consistent, well-configured sending resolves these automatically.
MailPilot’s steady, paced warmup sending pattern behaves like a legitimate sender, so transient issues like greylisting resolve on retry instead of hurting your domain.
See MailPilot’s sending behavior →