The best (in terms of power consumption and features) are in fact the
single-board computers. Even though it looks like the FSF does not approve of
those, if you read carefully the link Lembas provided, you will notice that
most of the time is because the graphics card requires non-free drivers and
such... Which is of no problem, since you will only run an email server, you
don't need 3d graphics or anything, so you can run those using ONLY free
software.
I have no experience with those machines, so, I can't really say which one is
better. But I remember reading that the beagleboards could run on a really
low power consumption. Take a look at the link lembas provided and
investigate the ones that the FSF considers can run free software but needs
non-free for wifi and graphics. You can add an external wifi later on
(thinkpenguin has those that are compatible with free software).
Hope it helps.
And congratulations on running your own email server =D Wish I could do the
same. You are much better off that way.
