Best practices when "splitting" a Virtualmin server - email, DNS...

I have Virtualmin GPL running almost a hundred domains on a fairly generic Ubuntu 10.04 (Lucid) Rackspace unit and in the natural course of events it is gradually becoming overloaded.

I have simply provisioned a new identical server and wish to move over some of the heaviest traffic domain sites just to that new server, while keeping the DNS on the old server. Of course, I'm looking at using virtualmin backup-domain/restore-domain to do this, and experiments on minor domains have been pretty encouraging, but now I'm trying to figure out the best way to do this systematically.

The "naive" way of doing this would seem to be the following:

1. backup-domain on the old server to a location on the new.
2. restore-domain on the new server. DNS still points to the old server.
3. test the domain on the new server.
4. switch the DNS on the old server to point to the new server.
5. test.
6. wait till everything has propagated, etc.
7. remove the domain on the old server.

Sounds logical, but in my experiments this didn't fully work out (and of course there's an issue with the last step which I never got to...)

Everything's good through step 4, which I accomplish by typing my best guess:

virtualmin modify-domain --shared-ip xx.xx.xxx.xxx --domain

Everything seems to work fine on the new server - except mail delivery. And now I think of it, I don't have a clear picture of how that would work... who's routing where?

Ideas? I'm going to do more experiments and report back.

If I did get to step 7, when I removed the domain from OLD using virtualmin, I'd also remove the DNS record, and the NEW site would suddenly go away.

I can hand-edit the BIND config files, which is certainly possible.

Ideas?

Thanks in advance!