My choice is now Apache James (http://james.apache.org/).
Tests went fine, things works as expected. Configuration is dull easy, like for those, who see an email server for the first time in their life. Apache James looks pretty good inside (source code), is extensible, has nice plugins and great API to make your own, if you like (that's exactly what I really need). Some folks reports it handles about 3K messages per a minute. I think, that's impressive number, but I don't need such anyways. :-)
The greatest thing of all this story: this particular mail server is a 100% pure Java.