Electronic mail is one of the Internet's "killer apps" and is arguably the most valuable in terms of end-user productivity. However, it was not always this way. In the early days of the Internet, there was no electronic mail service to speak of.
The first widely-used electronic mail service on the Internet was based on the file transfer protocol (FTP). Users would login to a system, upload a text file, retrieve any files that were waiting, and then disconnect. The process was streamlined in later years through the addition of a "MESSAGE" command to the FTP protocol, allowing for a modicum of automation.
The FTP approach was eventually deemed far too clumsy and resource dependent. Administrators had to create and manage user accounts on their systems for remote users, and the simple act of writing a one-line note involved many unnecessary steps (such as manually placing the message in a directory, setting the access privileges for that message, etc.). A new protocol was developed that allowed systems to exchange these messages seamlessly, called the Simple Mail Transport Protocol (SMTP).
It is important to note that SMTP only deals with how to exchange mail messages between sites; it does not define how the message body - or even the message envelope - should appear. This is why it is called the Simple Mail Transport Protocol.
Although it may seem obtuse to define separate standards for transport and content, both issues are sufficiently complex that they could not be dealt with simultaneously. Since there were a wide number of incompatible systems with incompatible networking protocols, the issue of how to move mail between these systems had to be dealt with on its own. Likewise, since there were an equally large number of incompatible mail systems in use on those divergent systems, the problems of how to format addresses, dates, and other headers had to be dealt with independently as well.
Apart from these twin issues of delivering appropriately formatted mail messages across different networks and mail systems, there was also a need for a standard method for retrieving messages from within mail systems. Since not all mail users would be running a local SMTP server, nor would they necessarily have access to a local mail system that supported TCP/IP, the Post Office Protocol (POP) was developed to provide these systems with access to electronic mail using TCP/IP protocols.
For more information on SMTP, refer to section D.2 Simple Mail Transport Protocol (SMTP). For more information about the POP3 mail protocol, refer to section D.3 Post Office Protocol v3 (POP3). For more information on the structure of SMTP mail messages, refer to Appendix E: Message Envelope, Header, Body and Parts.
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