Graceful way to move user's mail from one host to another?

[Adam Rosi-Kessel <adam@rosi-kessel.org>]

I run a small ISP with two hosts. I need to move some user accounts from one
system to another. There will be no DNS change--the accounts are just moving
from subdomain to another.

In the past when I've done this, I've always done an awkward one-by-one
move, where first I rsync the user's home directory over, then I move the
messages from the user's mail spool into a file in their home directory and
immediately put a .forward file in place, re-rsync the home directory, then
on the target system, I go and copy all the messages from the spool file in
their home directory back into the real spool file in /var/mail. (In cases
where I'm also changing DNS, I then implement the DNS change, and the
.forward file on the first host takes care of messages delivered to the old
IP address until the DNS change propagates).

This is the only way I could think of to insure that no mail is lost in the
short time while the transition is taking place.

Is there a more graceful, and ideally scalable, way to do this?

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