A popular e-mail messaging system from Microsoft that runs on Windows servers. The server side is Microsoft Exchange Server and the featured client program is Microsoft Outlook, which includes contacts and calendaring. Prior to Outlook, which was introduced with Office 97, the Windows client was Exchange Client. For the Mac, Outlook was available for many years, but was replaced by Entourage for Mac OS X. However, Outlook returned to the Mac in 2011. Other Mail Clients, Web and Mobile Outlook communicates with Exchange Server via Microsoft's MAPI protocol and can take advantage of all the Exchange features; however, Exchange Server supports global mailbox standards and can send to and receive mail from all POP3 and IMAP4 client programs. Exchange Server also supports Web-based access and smartphone access. See Outlook, Windows Live Mail and WINMAIL.DAT.