hMailServer is a free, open source mail server for Windows. It has support for SMTP, POP3, IMAP, virtual domains, anti virus scanning, distribution lists and much more. It comes with a great COM library that easily can be used for integration. Data is stored in a database server, MySQL or MS SQL, depending on your choice.

Features:

• POP3, SMTP, IMAP
• Virtual domains
• Built-in backup
• Scripting
• External accounts
• Rules
• Multilingual
• AntiVirus
• Spam protection
• Routing
• MX backup
• Mirroring
• Multihoming
• SQL-support

What's New in version 5.0 Build 321 Beta:

• It wasn't possible to display messages having an attachment with file names containing specific character.