MicrosoftExchange is the MicrosoftCorporation mail/collaboration/groupware system. It supports access through IMAP, POP3, NNTP, SMTP, its own proprietary MAPI protocol, and a webmail client (OWA, Outlook Web Access) which can be accessed both directly by a WebBrowser and by other programs, by means of a DAV interface. The latter is used by Ximian's commercial connector for Evolution to provide native access to Exchange. This is handy if the Exchange SysAdmin has disabled the internet protocols, or if you want to use Exchange's calendaring under Linux.

DBMail is a project to eventually build a OpenSource Exchange workalike. In the meantime, there are several HOWTOs on the web describing how to implement similar services to Exchange (shared calendaring, resource management etc).

Microsoft Exchange has built a notoriety for losing data -- and since it stores its data in a binary database, disaster recovery is particularly fun.