This is ProcMail, the ultimate incoming mail processor.
If you want it to replace the functionality of /usr/libexec/mail.local
then it must be suid root and needs the following lines in your
hostname.mc file for sendmail:

define(`LOCAL_MAILER_PATH', /usr/local/bin/procmail)dnl
define(`LOCAL_MAILER_FLAGS', ShPfn)dnl
define(`LOCAL_MAILER_ARGS', procmail -a $h -d $u)dnl
mailer(local)dnl