Decimail Server is now available for download exclusively using Subversion, so you'll need a Subversion client program if you don't already have one. To learn more about Subversion, visit http://subversion.tigris.org. The Decimail Server subversion repository is located at http://svn.decimail.org/decimail/; the latest development version (which may not work!) is at http://svn.decimail.org/decimail/trunk/ and releases can be found under http://svn.decimail.org/decimail/tags/releases/. To get release 0.15 using the standard subversion command-line client, do something like this:
(If you need to get an old version of Decimail Server you'll probably find that it will not work smoothly direct from Subversion. Do use the latest version if you can, otherwise get in touch for advice.)
To hear about new versions I suggest subscribing to decimail server's Freshmeat entry.