Glossary
Soft bounce
Last reviewed 18 September 2026
What is a soft bounce?
A soft bounce is a temporary refusal of a message by the receiving server, carried in a 4xx SMTP response, because the mailbox is full, the server is greylisting you, or you are being rate-limited.
It is retried with backoff rather than suppressed, and reclassified if it keeps happening. There is no published figure for when a soft bounce becomes permanent, because it depends on the reason code, but a common shape is to retry over a day or two and then treat somewhere between three and seven consecutive soft failures across a week or two as a hard bounce.
A mailbox that has been full for a fortnight is not a reader. A rise in soft bounces at one provider and nowhere else is also worth reading as a message about you rather than about the mailboxes, because that is what throttling looks like from the sending side.
See also
- Hard bounce
A hard bounce is a permanent refusal of a message by the receiving server, carried in a 5xx SMTP response, because the mailbox does not exist, the domain does not exist, or the address has been disabled.
- Throttling
Throttling is a receiving provider deliberately slowing or temporarily refusing mail from a sender, usually with a 4xx response, because the volume, the pattern or the reputation it is seeing is more than it is willing to accept right now.
- Delivery rate
A delivery rate is the share of a send that the receiving mail server accepted instead of bouncing, and it says nothing at all about whether any of it reached an inbox.
Read next
This term is covered properly in Email deliverability for small senders: authentication and reputation, which is a longer piece on the subject.
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