Sendmail, 3rd Edition

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 such as mush, elm, and mh; and commercial programs such as Zmail. Examples of MUAs also exist for PCs. Eudora and ClarisWorks are two standalone MUAs. Netscape and Explorer are web browsers that can also act as MUAs. 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. Usually, there is only one MTA on a machine. The sendmail program is an MTA.

Beginning with V8.10, sendmail also recognizes the role of a mail submission agent (MSA), as defined in RFC2476. MTAs are not supposed to alter an email’s text, except to add Received:, Return-Path:, and other required headers. Email submitted by an MUA might require more modification than is legal for an MTA to perform, so the new role of an MSA was created. An MSA accepts messages from an MUA, and has the legal right to heavily add to, subtract from, and screen or alter all such email. An MSA, for example, can ensure that all hostnames are fully qualified, and that headers, ...