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 corporate internal MAPI.DLL enabling these particular clients to talk to Xenix-based mail transport systems utilised this functionality. The first and almost immediately recalled version of the Microsoft Press book on using Microsoft Mail accidentally used screenshots from the Xenix backend, revealing this concealed capability of the client.