Network Administration: Linux E-Mail Basics
E-mail consists of messages that are delivered according to an Internet protocol commonly referred to as SMTP. SMTP, which stands for Simple Mail Transfer Protocol, was first codified in 1983, long before Al Gore invented the Internet. Several enhancements have been made along the way, but most e-mail on the Internet today is delivered using this nearly ancient protocol.
Interestingly, the software that delivers 70 percent of all the e-mail on the Internet — Sendmail — also originated in the same year. In 1983, Eric Allman developed the first version of the Sendmail program as part of the Berkeley Software Distribution (BSD) of Unix, one of the earliest versions of Unix made publicly available. Sendmail, is a standard part of most Linux distributions.
The following paragraphs describe some of the key features of e-mail that you should know about if you plan on setting up a Linux server running Sendmail:
Mailbox: A mailbox is a simple text file that holds incoming e-mail messages until they are processed by a mail user agent. In Fedora Linux, each user has a mailbox file in /var/mail.
Mail User Agent (MUA): A program that end users can use to send and receive mail. The most widely used MUA is Microsoft Outlook. Linux comes with several MUAs. The most basic is Mail, a text-based MUA that lets you read and compose e-mail messages from a console prompt.
Fedora also includes a sophisticated graphical MUA called Evolution, which is similar in many ways to Microsoft Outlook. Both are described later in this chapter.
Mail Transfer Agent (MTA): A program that transfers e-mail messages between computers. Sendmail, which most of this chapter is devoted to, is an MTA. When a user uses an MUA to send an e-mail message, the MUA delivers the message to an MTA, which then transfers the message to the intended recipient.
Mail Delivery Agent (MDA): A program that accepts incoming mail from an MTA and places it in the intended recipient’s mailbox. A basic MDA simply copies each message to the mailbox, but more advanced MDAs can be used to filter the incoming mail to eliminate spam or check for viruses.
The default MDA for Fedora Linux is Procmail. Fedora also includes SpamAssassin, which you can use to filter spam from your incoming mail.