Microsoft Mail Server (part of the Microsoft BackOffice family) includes Microsoft SQL Server, client-sever database, mail and more.
The second Microsoft Mail product was introduced in 1991 for PC Networks. It was based on Network Courier, a product that Microsoft purchased and updated. There were clients for MS-DOS and Windows. A stripped-down version of the PC-based server, Microsoft Mail for PC Networks, was included in Windows 95 and Windows NT 4.0. The last version was 3.5 as it was replaced by Microsoft Exchange Server (which starts with version 4.0).
The client software was also named Microsoft Mail and was included in some older versions of Microsoft Office such as version 4.x. The original "Inbox" (Exchange client or Windows Messaging) of Windows 95 also had the capability to connect to an MS Mail server. Later, they were replaced with the much more powerful Microsoft Exchange and later Outlook (Windows) and Entourage (Mac).
Microsoft Mail was a shared-file mail system; the "postoffice" was a database of files. Clients used mapped network drives and file sharing to write mail to the postoffice. Clients were in effect Message Transfer Agents (MTAs) for their own postoffices. Mail that needed to travel between postoffices required an external MTA. This MTA was called External.exe and ran on MS-DOS or Windows NT. A multitasking MTA was added with Microsoft Mail for PC Networks 3.5. This ran in the OS/2 subsystem of Windows NT and Windows 2000.