Even after a message is delivered to the final recipient, later recipient actions can alter the eventual disposition of that message. The recipient can choose to delete the message without reading it, read the message but not reply to it, forward the message, or do any number of other things to it. The Disposition-Notification-To: header was devised as a way to notify the sender about the ultimate disposition of the message. This header is advisory only, not mandatory, and is used like this:

Disposition-Notification-To: 1#address

Here, the 1# is literal. The domain part of the address is compared to the domain part of the address in the Return-Path: header (Return-Path:), and if they differ, or if the Return-Path: header is absent, no disposition notice is sent. If the two domains are the same, and if the recipient allows the response, notification of the message disposition is mailed back to the address using a special format.

See RFC2298 for a complete description of this header and the methods used to convey disposition notification.

The Disposition-Notification-To: header should never be declared in the configuration file.