Delivery Status Notifications (DSN)
(RFC 1891)

In the original SMTP, there was a mechanism for notifying of failures in mail delivery. However this mechanism was discovered as insufficient.

• The messages were insufficient to diagnose problems
• Sometimes it was impossible to decide where problems occurred
• Lack of standardization made it difficult for mail systems to handle the notifications messages.

The DSN extension should solve these issues and more.

How it works
After sending the EHLO command, if the DSN appears in the response, it means that the server supports this extension.
If so, 2 optional parameters are added to the RCPT command and to the MAIL command.

Extended RCPT command

• NOTIFY – used to specify the conditions for the generation of the delivery status notification.
• ORCPT – used to specify the actual/original recipient address that is specified by the sender.

Extended MAIL command

• RES – used to determine what information is returned to the sender in case of failure.
• ENVID – used to specify a special identifier that is recognized by the sender, and that will be contained in any delivery status notifications issued for the specific transmission.