A Delivery Status Notification (DSN), 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. Errors can occur at multiple places in mail delivery. A sender may sometimes receive a bounce message from the sender's mail server, and other times from a recipient's mail server. With the rise in forged spam and email viruses, users now frequently receive erroneous bounce messages sent in response to messages they never actually sent.
There are two types of DSN messages: transient and permanent. A transient DSN is generated when the server couldn't temporarily deliver the message. If delivery continues to fail after multiple attempts, a permanent failure DSN is generated.