As with regular postal mail, a properly functioning electronic-mail system depends on a series of distinct and often geographically-separated facilities and processes working together. Typically, each of these parts is handled by one or more programs specifically designed to perform the corresponding tasks.
In general, on Unix systems, the electronic mail facility is composed of the following components:
In the jargon, such programs are known as mail
user agents . There are a variety of such programs available,
ranging from the traditional (and primitive)
pine, and the
mh family, to Internet-integrated
packages such as Netscape (some users also prefer the mail
facilities embedded within their favorite editor, such as
emacs). These programs require only a
little administrative time and attention, usually consisting of
setting system-wide defaults for the various packages.
Delivering mail to its final destination is the responsibility of mail transport agents , which relay mail messages within a site or out onto the Internet toward their final destinations. Transport agents run as daemons, and they generally use the directory /var/spool/mqueue as a work queue to hold messages waiting for processing.
sendmail is the traditional ...