Personal need, but I'm sure I am not alone - just need to have WebMin/Virtualmin be *aware* that my mail is on a separate server - i.e. allow it to configure the DNS record with the appropriate pointer to the alternate mailbox' URI and IP address. That's pretty much all I would need, right now.
If that can be set on a per domain basis, I'd be pretty happy.
I do imagine that folks with larger or more mission critical installations would prefer to break out some of the functionality to different servers - thus, a separate mail server, and maybe even a separate MySQL server. If there were some way to easily accomodate that in Virtualmin (including interbox communication, so the main instance of Virtualmin could 'talk' to the mail server and the SQL server to create the appropriate settings, that would be even cooler.
Qmail support, in such a manner, would even be best (simply because I'm setting everything up with QmailToaster, which is a very easy installation, and offers a great deal of features and functionality built-in)
But, as I said, per domain/server settings to have Virtualmin configure different IP and MX for an outside server would be ideal right now.