Yes, you can. However, you must use version 5.1 or earlier. MAPI is not supported in version 6.0 and later. As well as installing WorkgroupMail 5.x as a server on a single gateway machine, you must also install it as a MAPI service on each client machine. You won’t be able to use it in conjunction with Outlook Express which is solely a POP3 client and relies on a TCP/IP network, however you will be able to use it in conjunction with any MAPI-enabled client, such as Microsoft Outlook or Windows Messaging.