small, silent "server"

Here's one page you might want to see
https://www.fsf.org/resources/hw/single-board-computers

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.

think pinguin provides the type of computer you like.

i find acer aspire one netbook zg5 to be good ssd with linpus preinstalled and available second hand. never used it as home server.
pleaselet us know howyou get on.