Whilst not wishing to add to your worries, let me say it is insane to rely on the DH email system for anything serious. There are just so many ways it loses mail. It only takes a customer to go on holiday and let his mailbox overfill, or gets a virus/spam attack, and the system can throw away all further incoming messages (no, not bounce them). I once lost an entire mailbox-full when a bug in the panel erased instead of edited the mailbox. Last week the Junk Filter was discarding messages because, according to Support, an essential server script wasn't running.
DH are smart people and I'm sure they'll get around to fixing the mail system up some time, but until then, beware.