By default, Redhat & Fedora comes with smrsh activated.
In order to forward an email to a handler program, the handler program must be in /etc/smrsh (I am not sure if link is sufficient)
Refer to /etc/mail/sendmail.mc
Below line means that smrsh is used.
FEATURE(`smrsh',`/usr/sbin/smrsh')dnl
/etc/aliases
localemail: |/etc/smrsh/mailhandler.sh
run newaliases to update the aliases db.
Also the /etc/smrsh/mailhandler.sh should read the input from the STDIN.