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 MicrosoftExchange. This is handy if the MicrosoftExchange SysAdmin has disabled the internet protocols, or if you want to use MicrosoftExchange's calendaring under Linux.

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

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

GreigMcGill calls it a typical pile of Microsoft shit, disguised as a group collaboration/mail server: 10/10 for features, -several million for implementation.