the same with dovecot works nicely
you can do this very easily with dovecot :
change ispconfig so that it prefixes with [DOMAIN]_
then change the line in the /etc/dovecot.conf file
next of course make sure all you're userid's are lowercase (otherwise just juse ="%Ld_%n") but the first solution avoids any typos in the login.
the nice thing is that squirrelmail works like a breeze,