This procmail recipe will answer anybody who adds an "X-Ack: yes" header line, confirming that their mail was processed by your procmail system (of course, it doesn't mean you've seen it, yet). In fact, only the "X_Ack:" header is required (but by changing the first regexp line, this can easily be modified). You should initialise MY_MAIL_ADDR to your email address, e.g. by saying "MY_MAIL_ADDR=username@mail.server.net". Also, as usual SENDMAIL should be initialised to point to your sendmail program.
# Generate receipts to all who request it!
:0 Hc
* ^X-Ack:
* !^FROM_DAEMON
* !$^X-Loop: $MY_MAIL_ADDR
| (formail -r -A"Precedence: bulk" -A"X-loop: $MY_MAIL_ADDR" ; echo "Ac\
knowledged, as per your request") | $SENDMAIL -t