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.