AfterLogic XMail Server Lite provides SMTP/POP3 services, AJAX webmail, multiple domains and accounts, virtual domains, spam filter, and lots of other features.
AfterLogic XMail Server Lite is
... [More]
distributed under GPLv2 license and is available for Linux (Unix) and Windows. Windows edition comes with ASP.NET webmail/webadmin frontend, Linux edition includes PHP version of these components.
AfterLogic XMail Server Lite page can be found at: http://www.afterlogic.com/products/xmail-server-lite
The commercial version with extended functionality (AfterLogic XMail Server Pro) is also available at: http://www.afterlogic.com/products/xmail-server-pro [Less]