This is not a sendmail issue. That said, there are recipes included in your systems documentation. See `man procmailex' for the details. Here is what a couple of the look like:
Example #1:
:0 h c
* !^FROM_DAEMON
* !^X-Loop: your@own.mail.address
| (formail -r -I"Precedence: junk" \
-A"X-Loop: your@own.mail.address" ; \
echo "Mail received.") | $SENDMAIL -t
Example #2:
:0 Whc: vacation.lock
# Perform a quick check to see if the mail was addressed to us
* $^To:.*\<$\LOGNAME\>
# Don't reply to daemons and mailinglists
* !^FROM_DAEMON
# Mail loops are evil
* !^X-Loop: your@own.mail.address
| formail -rD 8192 vacation.cache
:0 ehc # if the name was not in the cache
| (formail -rI"Precedence: junk" \
-A"X-Loop: your@own.mail.address" ; \
echo "I received your mail,"; \
echo "but I won't be back until Monday."; \
echo "-- "; cat $HOME/.signature \
) | $SENDMAIL -oi -t
|