were moved by an external MTA called External (external.exe), which originally ran on MS-DOS. A version of External for OS/2 1.31 was added with Microsoft Mail for PC Networks version 3.2, and a multitasking MTA for Windows NT was added with version 3.5. This ran in the OS/2 subsystem of Windows NT and Windows 2000, and consisted mostly of the Version 3.2a External with wrappers containing hooks allowing control by NT administration utilities. The database design and passive nature of the server both created several weakness. A complete lack of distinction between "header" and "envelope" addressing data meant that now-standard functionality such as Bcc: could not be implemented. Mailing lists were similarly problematic. Also, a single postoffice was limited to 500 mailboxes; a large enterprise would require many postoffices and many MTAs to connect these postoffices. Since scheduling data and address lists were also stored locally in these postoffices, moving schedule and global address book information required yet more, separate agents called Dispatch and Microsoft Schedule+ Distribution Agent. Dispatch would synchronise the various copies of the Global Address List using MSMail 3.x Directory Synchronization Protocol. Client Architecture Client handling of messages remained inconsistent through the life of the product. Clients for MS-DOS, dialup, and Mac OS left messages stored on server; the Windows and OS/2 clients downloaded the messages instead, using a local file (.MMF) as storage. While this storage could be located on a file server, the various file locking issues meant a user could only login to their mail from a single PC at a time. Further, the differing storage mechanisms made using MSMail across multiple client architectures problematic at best. While the inbox could be synchronised, other folders could not be, meaning that only inbox messages could be accessed across the two client architectures. Plans to create a Mac OS client which could access .MMF storage were abandoned before version 3.0 shipped, though working - if buggy - versions did exist. The Windows and OS/2 clients were built around an early version of MAPI called MAPI 0, and could in theory enable the client to talk to any server by replacing the MAPI DLL with a fully compliant DLL for the appropriate system. Unlike the Microsoft Mail 3.x server product, the clients could support Bcc: functionality. This functionality was not exposed to Microsoft Mail for PC Networks users due to limitations in the server architecture, but the