LumiSoft Mail Server is written in C# with source. The site says the program is freeware although it isn't clear what license the source is released under.

The site says it features:

• Supports multiple domains
• Supports multiple e-address for one mailbox
• Supports aliases(Mailing lists)
• Supports email routing. eg *ivar* pattern routes all addresses containing ivar to specified mailbox or remote address
• IP access filtering
• SMTP AUTH (LOGIN) (supported authentication types)
• SMTP SIZE, PIPELINING, 8BITMIME support
• POP3 APOP command
• POP3 remote accounts
• User mailbox size limit
• Full MS SQL server support
• Nice GUI for administation
• Well commented source included