We've looked for a year (off-and-on of course!) for a .NET implementation of the IMAP, POP, and SMTP protocols that left the "backend" unanswered (i.e., you "roll your own").  Well, we have finally found it with the WeOnlyDo(Software) implementation of these items!  Of course its not quite as good as all that - they are not native .NET, but ActiveX.  Still we're having a look and we'll advise on what we like and don't like about them!  Link is: http://www.weonlydo.com/index.asp?showform=ImapServer.

Also on our radar is a seemingly-nice little email server called MailEnable.  It has many features in the free version, but IMAP and Web come with the (paid) Professional and Enterprise versions.  We're going to test this one too!  www.mailenable.com