This is an example showing extraction of subject: and from: fields into variables, and running a command (say) before putting the mail into the default dir. The example is pretty much stolen from Timo Salmi
SUBJECT=`cat | egrep "^Subject:" | sed -re 's/^Subject: *//'`
FROM=`cat | egrep "^From:" | sed -re 's/^From: *([a-zA-Z0-9 @.])/\1/'`
:0c: # continue after this rule
* ^Subject:[ ].+[a-z0-9]
* ^From:[ ].+[a-z0-9]
| say "You've got mail from ${FROM} . Subject: ${SUBJECT}."
# Accept all the rest to your default mailbox
:0:
${DEFAULT}