Installation guide

Chapter 15. Email 211
particular mail server using the VRFY command or expand a mailing list using the EXPN command.
Email can also be relayed between two SMTP servers, if both systems permit such activity.
Unlike IMAP and POP, the SMTP protocol does not require authentication. This means that SMTP
servers can allow anyone on the Internet to use your system to send or relay mail to large lists of
recipients. It is this characteristic of SMTP that makes spam possible. Modern SMTP applications
attempt to minimize this behavior by restricting relaying and allowing only known hosts to send
email.
RFC-821 outlines the basic behavior of SMTP, but several SMTP extensions, made possible by RFC-
1869, have added additional functionality to SMTP over the years by making new commands avail-
able. By initiating a conversation with an SMTP server with an EHLO command rather than HELO,
the connecting server can identify itself as one that supports SMTP extensions. The receiving server
answers with a 250 line containing the various SMTP extensions it supports. Then, the connecting
server can use the supported extensions as it wishes to accomplish the goals of the communication.
One extension adds SMTP Authentication through the AUTH command as outlined in RFC-2554.
Another widely used SMTP extension, detailed in RFC-2034, discusses the use of dot-separated,
standardized error codes for use between SMTP applications. Reading the various RFCs that involve
SMTP provides a background to the way email moves around the Internet. In addition, you can con-
nect to an SMTP server via telnet by specifying port 25, such as telnet localhost 25. Executing
a few commands and sending a mail manually is a good way to understand how SMTP communica-
tions work.
Red Hat Linux 8.0 uses Sendmail (/usr/sbin/sendmail)as its SMTP program by default. How-
ever, an easier to use application called Postfix (/usr/sbin/postfix) is also available for use.
Section 15.2.2 for more on this topic.
15.2. Email Program Classifications
In general, all email applications fall into at least one of three classifications. Each of these classifica-
tions plays a specific role in the process of moving and managing email messages. While most users
are only aware of the specific email program they use to receive and send messages, each of these
types is important for making sure email arrives at the correct destination.
15.2.1. Mail User Agent
A Mail User Agent (MUA) is a program that, at the very least, allows a user to read and compose email
messages. An MUA is often referred to as an email client. Of course, many MUAs help users do more
than that, including retrieving messages via the POP or IMAP protocols, setting up mailboxes to store
messages, or helping present new messages to a Mail Transfer Agent that will deliver them to their
final destination.
MUA programs may be graphical, such as Mozilla Mail, or have a very simple, text-based interface,
such as mutt.
15.2.2. Mail Transfer Agent
A Mail Transfer Agent (MTA) transfers email messages between machines using SMTP. A message
may involve several MTAs as it moves to its intended destination. Most users are totally unaware of
the presence of MTAs, even though every email message is sent through at least one MTA.
While the delivery of messages between machines may seem rather straightforward, the entire process
of deciding if a particular MTA can or should accept a message for delivery is quite complicated. In
addition, due to problems from spam, use of a particular MTA is usually restricted by the MTA’s own
configuration or network access to the system running it.