expiring old mail from a mbx file

[Leonardo Boselli <leo@dicea.unifi.it>]

I need a script able to remove all messages delivered before a
certain date to a mailbox and still there. 
Some users use to do this with certain pop3 clients, so they
want absolutely the same function even if they shift to reading e-mail
with squirrelmail.
(they save "important" messages in differnt folder, but leave
"minor" messages in the inbox so they can access again fron a different
place if necessary, but such " forgotten" fessages must be cleaned to
avid uncontrolled growth of mailbox)