A bounce message or just “bounce” is an automated message from a mail system, informing the sender of a previous message that that message had not been delivered (or some other delivery problem occurred). The original message is said to have “bounced”.
More formal terms for bounce message include “Non-Delivery Report” or “Non-Delivery Receipt” (NDR), [Failed] “Delivery Status Notification” (DSN) message, or a “Non-Delivery Notification” (NDN).
Errors may occur at multiple places in mail delivery. A sender may sometimes receive a bounce message from their own mail server, reporting that it has been unable to send a message, or alternatively from a recipient’s mail server reporting that although it had accepted the message, it is unable to deliver it to the specified user.
When a server accepts a message for delivery, it is also accepting the responsibility to deliver a bounce message in the event that delivery fails.