Microsoft Exchange Server is a mail server and calendaring server developed by Microsoft. It runs exclusively on Windows Server operating systems.
Exchange Server was initially Microsoft’s internal mail server. The first version of Exchange Server to be published outside 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.
Exchange Server is licensed both in the forms of on-premises software and software as a service. In the on-premises form, customer purchaseclient access licenses (CALs). In the software as a service form, Microsoft receives a monthly service fee instead (see Microsoft Office 365).