HMailServer is a free, GNU GPL licensed email server for Windows. It runs as a service with a management tool that connects to allow configuration of settings similar to the way Exchange works. It has support for the IMAP, POP3, and SMTP open email standard protocols, and stores index data in MySQL or MS SQL but the actual e-mail content is still stored as files in a format similar to Maildir (the contents are the same but the directory structure is different). It is updated frequently and has active support and development forums.

All common features such as multiple-domain support, aliases (catch-all and specific ones for addresses and also for domains themselves) and basic mailing lists are present. Users can be authenticated against both the local system and other servers, including domain controllers.

It offers a variety of anti-spam options such as:-

• Host based DNS Blacklisting (DNSBL)
• URL based DNS blacklisting (SURBL)
• Greylisting (must retry sending for the message to succeed)

And provides basic support for using JScript or VBScript to modify the behaviour of the server on receipt of messages and at a variety of other events (including client connection).

HMailServer in current versions does not include SSL or TLS support, however this can be easily added using a third party application called STunnel which uses the OpenSSL libraries to add secure connection support. IPv6 support can also be added using STunnel, it is not included.

There is recent discussion that further releases of the software will not be open source visible on the forums by the developer.

Some features that may be of interest to businesses but are commonly missing in other servers are also available and easy to configure, such as:-

• Compulsory signatures (for legal and advertising footers)
• Server side rules
• External account retrieval
• Quotas on domain, mailbox, and individual message sizes
• Plus addressing (using + to make a virtual alias for an account specific to a task, as seen in Gmail)
• Attachment blocking
• E-Mail mirroring (all messages sent to a particular address)
• Custom SMTP routes for specific domains

Integration is supported for many related applications, including:-

• SquirrelMail for webmail, (requires IIS or Apache); spell-checking available
• ClamAV anti-virus software
• SpamAssassin spam filtering
• and more…