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Chapter 15. Email

Many of the larger and more complex MUAs can also be used to send email. However, this action

should not be confused with the actions of a true MTA. In order for users not running their own MTA

to move outbound messages off of their machine and onto a remote machine for delivery, they must

use a capacity in the MUA that transfers the message to an MTA they are authorized to use. However,

the MUA does not directly deliver the message to the intended recipient's email server   that role is

reserved for the MTA.

Red Hat Linux uses Sendmail as its default MTA, though others may be used in its place.

Tip

For information about how to switch the default MTA from Sendmail to Postfix, see the chapter called

Mail Transport Agent (MTA) Configuration in the Official Red Hat Linux Customization Guide.

15.2.3. Mail Delivery Agent

A Mail Delivery Agent (MDA) is used by the MTA to deliver email to a particular user's mailbox. In

many cases, an MDA is actually a Local Delivery Agent (LDA), such as

/bin/mail

or

procmail

.

However, Sendmail can also play the role of an MDA, such as when it accepts a message for a local

user and appends it to their email spool file. Any program that actually handles a message for delivery

to the point where it can be read by an MUA can be considered an MDA. Note that MDAs do not

transport messages between systems or interface with the end user.

Many users do not directly use MDAs, because only MTAs and MUAs are necessary to send and

receive email. However, some MDAs may be used to sort messages before they are read by a user,

which is a big help if you receive a lot of email.

15.3. Sendmail

Red Hat Linux uses Sendmail as its default MTA, whether they are intended for users on the same

system or for remote destinations. Other MTAs do exist, but many system administrators elect to use

Sendmail as their MTA due to its power, scalability, and compliance to important Internet standards,

such as SMTP.

Sendmail's core duty, like other MTAs, is to safely move email between hosts, usually using the SMTP

protocol. However, Sendmail is highly configurable, allowing you to control almost every aspect of

how email is handled, including the protocol used.

15.3.1. History

Sendmail's roots can be traced to the birth of email, occurring in the decade before the birth of

ARPANET, the precursor to the Internet. In those days, every user's mailbox was a file that only

they had rights to read, and mail applications simply added text to that file. Every user had to wade

through their mail file to find any old mail, and reading new mail was a equally difficult. The first ac 

tual transfer of a mail message file from one host to another did not take place until 1972, when email

began to be moved by FTP over the NCP network protocol. This easier method of communication

quickly became popular, even to the point where it made up most of ARPANET's traffic in less than a

year.

However, a lack of standardization between competing protocols made email much harder to send

from some systems. This continued until the ARPANET standardized on TCP/IP in 1982. Soon after,

a new protocol, SMTP, materialized for transporting email messages. These developments, combined

with HOSTS files being replaced with DNS, allowed full featured MTAs to materialize. Sendmail,