A bounce message indicates that the email could not be delivered successfully. Usually libraries for sending email allow to set a separate bounce address which receives the bounced messages.
In the Internet's standard e-mail protocol SMTP, a bounce message, also called a Non-Delivery Report/Receipt (NDR), a (failed) Delivery Status Notification (DSN) message, a Non-Delivery Notification (NDN) or simply a bounce, is an automated electronic mail message from a mail system informing the sender of another message about a delivery problem. The original message is said to have bounced.
See Wikipedia for details.
