Assuming your using procmail with your MDA and maildir format, simply edit /etc/procmailrc and add:
Code:
MAILDIR=$HOME/Maildir/
DEFAULT=$MAILDIR
obviously, replace Maildir with the actual maildir name in the users home directory (if that isn't it).
ETA, if the mail dir doesn't exist yet, run maildirmake Maildir with the user (ie, not as root)