Bounce codes are automatic messages sent from a mail system to inform you of a delivery problem. These messages are called non-delivery reports (NDRs), or bounces. Bounce reports may be returned synchronously in the course of the Simple Mail Transfer Protocol (SMTP) conversation or asynchronously through a separate email conversation.
Bounce codes are typically returned if:
- The email is blocked
- The email address is not valid (also called an unknown user)
- The email is being throttled
- The recipient's mailbox is full
As a sender, you need to properly identify the reason for bounces by reviewing the codes and and acting on them accordingly. Failure to handle bounces correctly can lead to serious reputation and deliverability problems.