We tested six free or open source email server products: Apache James (cross-platform), Citadel (Linux-only), hMailServer (Windows-only), SmarterMail (Windows-only), Zarafa (Linux-only) and Axigen (cross-platform). Most have commercial options, but we specifically limited our testing to the free or community editions.
Free email servers for small businesses
Pros: Unlimited use, easy to install and configure, flexible code base runs on most Windows versions.
Cons: Lack of native mobile and web support.
Pros: Fully open source, no limits on users, small footprint, easy to install.
Cons: Dated GUI, may not scale well for larger organisations.
Pros: Nice user interface, good support for mobile mail, informative tutorials.
Cons: Free version limited to 10 users on a single domain.
Pros: Java platform runs on both Linux and Windows, 'mailets' provide modular design and flexibility.
Cons: Current stable release dated August 2009; no GUI.
Pros: Cross-platform, great user interface, easy to install, includes help features.
Cons: Free version limited to 100 users.
Pros: Comprehensive mobile support, pleasing user interface, LDAP integration.
Cons: Community edition somewhat lacking in features, installation has quite a few prerequisites.
Linux creator Linus Torvalds wants comments on a proposal for an entirely bug-fixes and stability release in about a year
Improving security doesn't always mean having to buy more equipment or invest in expensive training