NMail

NMail is a messaging and collaborative software server written in C#. It is written to support open standards such as IMAPv4 and SMTP. It is licensed under the Apache 2 license.
Features:
- 100% .NET implementation.
- A SMTP client and server.
- An IMAP server.
- A Windows installer and setup wizard.
- ASP.net Webmail.
- An ASP.net administration site.

Go to http://nmailserver.sourceforge.net/index.php/Main_Page

See also these related projects.