Sendmail - Red Hat LINUX 7.2 Reference Manual

Section 16.3:Sendmail 263
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.
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. It is important
that you disable the currently running MTA before starting up another one, as both will try to claim
port 25, the default SMTP port.
16.2.3 Mail Delivery Agent
A Mail Delivery Agent (MDA) is utilized 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 utilize 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.

16.3 Sendmail

Red Hat Linux uses Sendmail as its MTA to deliver messages, whether they are intended for users on
the same system or for remote destinations. Other MTAs do exist (and can be used well with Red Hat
Linux), but most 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 utilizing the
SMTP protocol. However, Sendmail is highly configurable, allowing you to control almost every
aspect of how email is handled, including the protocol to be used.
16.3.1 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 chore. The first actual transfer