2 Requests for Comments (RFCs)
A complete understanding of sendmail is not possible without at least some exposure
to Requests for Comments (RFCs) issued by the Internet Engineering Task
Force (IETF) at the Network Information Center (NIC). These numbered documents
define (among other things) the Simple Mail Transfer Protocol (SMTP) and
the format of email message headers.
When you see a reference to an RFC in this book, it will appear, for example, as
RFC2821. The RFCs of interest to sendmail are listed in the Bibliography at the end
of this book.
1.3 Email and sendmail
A mail user agent (MUA) is any of the many programs that users run to read, reply
to, compose, and dispose of email. Examples of an MUA include the original Unix
mail program (/bin/mail); the Berkeley Mail program; its System V equivalent
(mailx); free software programs suchas mush, elm, pine, and mh; and commercial
programs suchas Zmail. Examples of MUAs also exist for PCs. Eudora and Claris-
Works are two standalone MUAs. Netscape and Explorer are web browsers that can
also act as MUAs. Thunderbird is an open source MUA from the folks at Mozilla.
Many MUAs can exist on a single machine. MUAs sometimes perform limited mail
transport, but this is usually a very complex task for which they are not suited. We
won??™t be covering MUAs in this book.
A mail transfer agent (MTA) is a highly specialized program that delivers mail and
transports it between machines, like the post office does.
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