A short while ago I purchased a license for an Email server called EmailArchitect by Admin Systems.  I can't say how impressed and pleased I am with the software.  While there are a few very minor things that I still take issue with (mostly because I'm such a hard-nosed perfectionist), I very much like the software.

I had an interesting experience with them this week that I'd like to relate.  I sent an email request to their support on Tuesday expressing a few issues (mostly typos, and the like) and I got an email back later that night indicating that they'd look into it and make the corrections in their next version.  At the time, I was running Email Architect 6.0.1.1.  Right now, I'm running 6.0.1.2 - two and a half days later and they have incorporated all of the changes that I requested and published it on their website!

That's awesome!  If you're in the market for a great email server application, check 'em out!