I have recently discovered when migrating some mailboxes to postfixadmin, that schema created by 2.3.5 installer doesn't have any foreign keys (except of on_vacation). This at least causes some caveeats when doing some manual modifications/imports into database. You can end up with mailboxes without domain and similar problems. I recommend adding foreign keys there.
I you want, I can create patch for this.
Please write me if you want it.