A soft bounce is a delivery failure that the receiving server reports as temporary. In SMTP, the reply code begins with 4, and the enhanced status code begins with 4 as well, for example 4.2.2 for a mailbox that is full.
Why it matters
Mail servers queue deferred messages and retry them for a period, often days, before giving up and generating a final failure notice. So a soft bounce usually causes a delay, not a loss. A pattern of them, though, can point to something fixable: a receiver throttling you for sending too fast, greylisting, or a reputation problem that has not yet become a block.
Example
451 4.7.1 Service unavailable, try again later
452 4.2.2 Mailbox full
421 4.7.0 Too many connections from your IPIf many messages to one provider defer, slow your sending rate and spread it over a longer time. If one recipient keeps deferring with a full mailbox, set a limit on how many attempts your platform makes before you pause mailing that address.
Common mistakes
- Suppressing addresses after one soft bounce, losing valid subscribers.
- Ignoring repeated deferrals from a single provider, which can precede blocks.
- Retrying too aggressively, so the receiver throttles you harder.
Related
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