Glossary · Email
Soft bounce
A temporary delivery failure, such as a full mailbox or a busy server, where a retry may succeed.
Soft bounces cover conditions that might resolve: the mailbox is over quota, the server is unavailable, or the message was greylisted. The address itself is usually real.
Sending systems retry soft bounces on a schedule and only give up after repeated failures. A mailbox that soft bounces for weeks is effectively abandoned even though it technically exists.
For list hygiene, repeated soft bounces are worth acting on. They usually mean the person stopped using the account rather than that the server had a bad day.