Microsoft Exchange Server is a mail server and calendaring server developed by Microsoft. It runs exclusively on Windows Server operating systems.
The first version of Exchange Server to be published by Microsoft was Exchange Server 4.0. Exchange initially used the X.400 directory service but switched to Active Directory later. Versions 4.0 and 5.0 came bundled with an email client called Microsoft Exchange Client. It was discontinued in favor of Microsoft Outlook.
Exchange Server uses a proprietary protocol called MAPI. Over time, however, it added support for POP3, IMAP, SMTP, and EAS.