Zarafa
The Zarafa Collaboration Platform is an open source e-mail and colloboration software suite which is the equivalent as a replacement for Microsoft Exchange and can be connected using native clients such as Outlook as well as open source products such as Thunderbird and Evolution and includes support for LDAP integration.
Zarafa also developed Z-push, which is an open source implementation of the Microsoft Exchange ActiveSync protocol. This allows you to always stay in sync with your mobile device, whether it's running Android, iOS, Blackberry OS or Symbian OS.
The platform wouldn't be complete without a powerful and intuitive web frontend, this is provided in the form of the Zarafa WebApp. This frontend (with HTML5) provides easy access to your mail and calendaring.
M:Tier engineers have performed the initial port to OpenBSD, and have been maintaining it in OpenBSD ever since. Zarafa is also used internally within M:Tier since 2009.
M:Tier has performed various upgrades and migration projects of existing e-mail solutions / infrastructure to the Zarafa platform.
Zarafa and OpenBSD are a secure combination which enables your organisation to provide appropriate mitigation against various kinds of network attacks.