[exim] Automatic /var/spool/mail cleaning

Hi!
I have set a new router to deliver a copy of my mails to an exchange
server using this settings:

exchange2003:
      driver = manualroute
      domains = +local_domains
      transport = remote_smtp
      route_list = * 192.168.123.254
      unseen = true

With this settings, I keep the mail on my Linux server, this is a good
thing since I have a little backup on my linux.

But I would like to know if there is a procedure to cleanup
/var/spool/mail/%users% by deleting mail older then 3 month for exemple

Hope someone got a solution,

regards