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E-mail involves application software that resides on a computer platform of some description. E-mail was popularized as early as the 1970s, as part of the office automation concept. Typically residing on a midrange computer, early systems included IBM PROFS (PRofessional OFfice System), DEC DECmail (included in the All-in-One applications suite), and the Wang Office. The proliferation of PCs and PC LANs in the 1980s stimulated the growth of e-mail, which then became affordable and, therefore, accessible by a much larger community of potential users.
The contemporary e-mail network often is in the form of a dedicated server, to which client workstations gain access for messaging purposesa client/server configuration. Clients, or user agents, are the workstations which create, transmit and receive the messages. They include software which effects compatibility with the server. Servers store messages and provide transport and directory services to the client. Various clients can be linked to a server through Application Programming Interfaces (APIs), messaging middleware (software which logically sits between client and server), and a driver package. Examples include Microsoft Messaging Application Programming Interface (MAPI) and Apple Computer Open Collaboration Environment (AOCE). Such APIs allow client applications such as word processing and spreadsheets to communicate with an e-mail server. In other cases, special drivers are required to access servers running such proprietary access protocols as DEC MailWorks 1.5 and MCIMail.
Servers may be in the form of a logically partitioned mainframe, mid-range, or PC computer platform. In large user organizations, the clients generally are LAN-attached. The server also may be provided on a service bureau basis, such as is provided by specialized carriers and online services including America Online, AT&T EasyLink, Australia Online, CompuServe, the Internet, MCI Mail, Prodigy, The Source, and The Well. Many client/server e-mail systems run equally effectively in several operating system environments, such as Windows, Macintosh, OS/2 and UNIX.
Networking
Although some e-mail networks are closed, most provide for Wide Area Networking (WAN networking) over a backbone, or centralized high-capacity network, in order to provide for message communications with the outside world. In any event, remote access generally is provided to employees, vendors, clients and others within the inner circle. Such islands are increasingly rare; servers typically support multiple protocols, with internetworking between disparate e-mail systems adding a great value to messaging technologies.
Protocols
Each e-mail network is governed by a set of protocols, which are the rules and conventions by which the network and component network elements, operate. Such protocols are embedded in the application software and the specific operating system (OS) or network operating system (NOS) which governs the manner in which the computer or computer network functions. Elements of the software that achieves compatibility between the client workstation and the server over the network generally resides on the client, as well as the server (Figure 5.4). E-mail protocols govern such characteristics as addressing conventions, routing instructions, message structure, and message transfer. Protocol examples include X.400, X.500, and various proprietary protocols [5-28].
Figure 5.4 Client/server e-mail, supporting multiple protocols.
X.400
X.400 is an ITU-T international protocol for e-mail document exchange. X.400 is compatible with the International Standards Organization (ISO) Open Systems Interconnection (OSI) model, functioning at Layer 7, the Application Layer. As depicted in Figure 5.5, X.400 allows disparate e-mail systems to interoperate at a minimal level, over either a X.25 packet nework or asynchronous dial-up circuits [5-29].
Figure 5.5 Networked e-mail (X.400).
X.500
X.500 is an international standard being developed jointly by the International Standards Organization (ISO) and ITU-T. X.500 provides for global directory services that theoretically allow network managers to store information about all users, machines, and applications in a distributed fashion. The current version of X.500 provides for directory replication functions that allow multiple copies of the directory information to be stored throughout the network, rather than residing on a centralized server. The current version also provides improved security through authentication access controls. Although X.500 has yet to be fully embraced by the software developers, Digital Equipment Corp., Isocor and others have incorporated X.500 into their proprietary e-mail products. A move currently is underway to place a X.500 directory on the Internet. Functioning like a telephone white pages, it would allow telephone and e-mail users to quickly find the addresses of other users, with certain security-oriented limitations [5-30], [5-31], [5-32], [5-33], and [5-34].
Proprietary protocols
Proprietary protocols include cc:Mail (Lotus Development), Lotus Notes (Lotus), Microsoft Mail (Microsoft), StreetTalk (Banyan Systems), BeyondMail (Banyan) and GroupWise (Novell). At the end of 1994 there were over 12 million users of the two most popular desktop e-mail systems, cc:Mail and Microsoft Mail; that is an increase of seven million over the previous year [5-35].
Through gateway technology, such as is in place on the Internet, mail systems that are not alike can interoperate. Gateway software acts as a protocol translator, or interpreter, converting from one native environment to another [5-36]. In this manner, cc:Mail and Microsoft Mail software can pass messages. Similarly, I was able to send drafts of this chapter and associated graphics from America Online to the publisher and editors; similarly, they were able to respond with edits until such time as the document was deemed satisfactory. To supplement this gateway process, various software available at no charge on the Internet was downloaded in order to preserve formatting (bold type, bullets, and left-justification).
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