You most probably could setup a system whereby the email is forwarded but not kept locally while still retaining the ability for them to use the outgoing smtp.. but it's definitely much simpler just to automatically remove old mail they don't pickup. Just use "find" in a simple script and run it daily with a cron. Script eg:
find /var/vmail/domain.name/mailbox1/new/* -mtime +2 -exec rm -f () \;
find /var/vmail/domain.name/mailbox2/new/* -mtime +2 -exec rm -f () \;
Run it via cron once a day and it will delete anything over 2 days old in that directory. I would leave a day or two's mail in case one day they phone up and say "You wouldn't have a copy of my mail from yesterday would you, I accidentally deleted it in my gmail". (You become a God to them when you answer yes to such questions).