Bounce

What it means

A bounce happens in one of two ways. The receiving server can refuse the message during the SMTP session with a 5xx reply, and the sending system then writes the bounce itself. Or a server accepts the message and only later finds it cannot deliver it, in which case RFC 5321 requires it to send a non-delivery notification to the envelope sender. A message deferred with 4xx replies becomes a bounce only when the sender finally gives up retrying.

Bounces are usually sorted by cause: hard bounces for addresses that will never work, soft bounces for temporary or mailbox-level problems, and block bounces for refusals based on the sender's reputation, authentication or content. The category decides the response: suppress the address, try again later, or fix something on the sending side.