How are bounce messages generated?

Bounce messages are generated when delivery fails after the message was initially accepted:

Generation process:

Sending server delivers message and receives acceptance (250 OK). Receiving server queues message for final delivery. Processing fails (user not found, quota exceeded, filtering). Receiving server creates a Delivery Status Notification (DSN). DSN is sent to the Return-Path (envelope sender) address.

DSN contents typically include:

Original message headers (sometimes full message). Error description and SMTP status code. Timestamp of the failure. Receiving server identification.

Who generates bounces:

The final destination server if the mailbox fails. Intermediate relays if they cannot forward. The sender's own server if it cannot connect.

Bounce messages are the postal service's return-to-sender notification. They travel back through the mail system to report the failure.