There are plenty of free and open source mail server solutions in the Linux and Unix world which based on Postfix, and there are also standalone suite such as Courier, Zimbra, etc.
However, most mail server solutions in Windows world are not free. Among those which are free also always lack in features. hMailServer, created by Martin Knafve, is a decent one in the Windows world, which is free to use and also provides all of the important functionality you need to run your own e-mail server. Initially it was open source and licensed under GPL, until recently its Version 5 is now closed source (source code no longer available) but is still provided free of charge to use.
hMailServer supports open email protocols such as IMAP, POP3 and SMTP, which makes it compatible with most email clients available, including webmail such as SquirrelMail. Among its features are:
- Multiple virtual domains
- Domain alias
- Antivirus (with ClamAV)
- Rules and filtering
- Attachment blocking
- Retrieval of messages from external POP3 accounts
- Built-in backup
If you are from the SME/SMI sector, looking for an email server solution to be installed in a Windows platform, hMailServer is a solution that is worthwhile for you to seriously look into.