Even with multimedia attachments and HTML encoding prevalent in e-mail messages today, the technology behind message transfer hasn't changed significantly since the early 1980s. The framework for the Simple Mail Transfer Protocol (SMTP) was initially described in RFC 821 in 1982. The protocol itself was extended in 1993 (RFC 1425), yielding the Extended Simple Mail Transfer Protocol (ESMTP), which provides more commands and new delivery modes.
The three parts to message transfer are the Mail Transfer Agent (MTA), the Mail Delivery Agent (MDA), and the Mail User Agent (MUA). The MTA, commonly referred to as the mail server (of which sendmail and postfix are examples), actually handles distributing outgoing mail and listening for incoming mail from the Internet. The MDA accepts messages from the MTA and copies the message into a user's mailbox. Red Hat Linux uses /usr/bin/procmail as the default MDA, as specified in sendmail's configuration file. For sites that have a centralized mail server, Post Office Protocol (POP) clients are also considered MDAs. An MUA is the program run by a user to read incoming mail or to send messages to others.
See Chapter 9 for details on Mail User Agents available with Red Hat Linux.
This chapter focuses on the sendmail MTA, the most common mail server on the Internet. Nearly 70 percent of all e-mail messages on the Internet are delivered by sendmail. With the growing Internet population, billions of e-mail messages are sent and received each day. As an alternative to sendmail, this chapter also includes a short description of postfix.
There have been three major releases of sendmail. The original sendmail (sendmail version 5) was written in 1983 by Eric Allman, a student at the University of California at Berkeley. He maintained the code until 1987, when Lennart L?vstrand enhanced the program, simplified the configuration, and developed IDA sendmail. Eric Allman returned to Berkeley in 1991 and embarked on a major code revision, releasing sendmail V8 in 1993, which incorporated the extensions from IDA sendmail. The current version (8.12.9) is based on this "version 8" code.