For many years I have used Mdaemon as my mailserver, its very easy to use, have great security and anti spam, have very little hardware requirements and a superb webmail client.
Only problem has been the price, this is however no longer an issue as Mdaemon now exist in a free 5 user version.
MDaemon Email Server software for Windows is now available in a FREE 5-user license perfect for home and SOHO users wanting a reliable mail server that will grow with their business.
Take it for a spin via;
http://www.altn.com/Products/Free-Windows-Mail-Server/
Screenshots;
http://www.altn.com/Products/MDaemon-Email-Server-Windows/ScreenShots/