By far the best chess server for atomic (and other chess variants) is FICS - Free Internet Chess Server. That's where i play as siggemannen as well. There are many active players in the range from beginners to very strong players, atomic stars like MillzesChess and szura, plus lots of new strong players. Right now atomic is gaining in popularity even more, which is a good thing in my opinion.

To visit FICS, go to http://www.freechess.org, click on Login. you can logon as guest and play atomic or anything you want. You can register to get ratings and other useful things, it's free. Same address, click on Register, and fill out the form.

Other servers are USCL which is a server where you must pay.
Also ICC is a server where you have to pay.

Atomic by mail sounds crazy? And it is, but there is a site with longer time controls (min 1 day): http://www.brainking.com

If you have any questions about those servers feel free to send me and email to: siggemannen@gmail.com