Publisher Summary Microsoft shipped Exchange 4.0 in March 1996 after a gestation period of some four years. The new messaging server went through many different design phases. The messaging technology has made enormous progress since 1996. Exchange has improved its capabilities in terms of: functionality, robustness, security, and connectivity. The web is the best and most pervasive example of a technology that has influenced Exchange. Exchange 2007 introduced messaging application programming interface (MAPI).Net—a modern version of the server-side MAPI that Exchange uses for communication between servers. Microsoft tuned the server variation of MAPI in Exchange 2003 to better support the kind of multi-threaded applications that servers run. Exchange 2007 supports a broad range of Internet protocols like POP3 and IMAP4 on the client side to SMTP—as the basis for messaging connectivity and transport, to HTTP for Web access, and extensions that provide better security and functionality—like ESMTP and HTTPS. The Outlook Web Access (OWA) version that shipped with Exchange 2000 marks a dramatic step forward in the user interface (UI) and performance, and has become a client that can be useful as a replacement for Outlook.