>>>>> "KG" == Kai Großjohann <Kai.Grossjohann@CS.Uni-Dortmund.DE> writes:
    >>  This is actually not more than a header in the mail, and I had
    >> seen somewhere in the changelog of Gnus that a function
    >> inserting it had already been introduced a few month ago.

    KG> Hm.  The code I saw invoked sendmail with various options,
    KG> depending on some Emacs variables.  No message headers
    KG> appeared to be involved.

    KG> If it is possible to speficy that I want DSN by including a
    KG> header in the outgoing message, then surely Gnus should use
    KG> that, rather than kludging in the DSN command line arguments
    KG> when invoking sendmail.  Right?

Matthieu Moy pointed me to the following entry in the changelog:
2002-01-05  Simon Josefsson  <jas@extundo.com>
            (snip)
        (message-insert-disposition-notification-to): New function.

According to the manual, this is:
*Disposition Notifications - RFC 2298*
     Message Mode is able to request notifications from the receiver.
     (and)
`C-c M-n'
     Insert a request for a disposition notification.
     (`message-insert-disposition-notification-to').  This means that
     if the recipient support RFC 2298 she might send you a
     notification that she received the message.


Disposition-Notification-To, according to RFC 2298, is implemented by
inserting a standard RFC822 header into the message. It is therefore
the responsibility of the MUA.

Conversely, according to [0], DSN can be requested from sendmail
either in a commandline argument or in the SMTP conversation. I don't
think it can be activated by inserting a header -- DSN is invoked at
the mail transport level, not by the message content.

--Joe

[0] http://www.sendmail.org/~ca/email/sm8.8.new.html