Glossary

Bounce

A message that could not be delivered and was returned to the sender with a reason.

ValidReach Team · Updated 2026-10-26

In practice

A hard bounce is permanent, for example the mailbox does not exist (SMTP 5xx codes). A soft bounce is temporary, for example the mailbox is full or the server is busy (4xx codes). Retry soft bounces a few times; remove hard bounces immediately.

Example

A message to ghost@northwind.example.test is rejected with “user unknown”: that is a hard bounce.

Verification and sending

Verification predicts hard bounces before you send. An address marked invalid is one that would have bounced.