Mail Server
Components – MTA , MDA & MUA
Overview:
In general, all email servers
consists of three main components: MTA , MDA & MUA. Each components
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 one is important for ensuring that email
arrives at the correct destination.
Mail
Transport Agent(MTA)
A Mail Transport Agent (MTA)
transports email messages between hosts using SMTP. A message may involve
several MTAs as it moves to its intended destination.
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 configuration or the access
configuration for the network on which the MTA resides.
Many
modern email client programs can act as an MTA when sending email. However,
this action should not be confused with the role of a true MTA. The sole reason
email client programs are capable of sending email like an MTA is because the
host running the application does not have its own MTA. This is particularly
true for email client programs on non-UNIX-based operating systems. However,
these client programs only send outbound messages to an MTA they are authorized
to use and do not directly deliver the message to the intended recipient’s
email server.
Since
Red Hat Enterprise Linux offers two MTAs, Postfix and Sendmail, email client
programs are often not required to act as an MTA. Red Hat Enterprise Linux also
includes a special purpose MTA called Fetchmail.
Mail
Delivery Agent (MDA)
A
Mail Delivery Agent (MDA) is invoked by the MTA to file incoming email in the
proper user’s mailbox. In many cases, the MDA is actually a Local Delivery
Agent (LDA), such as mail or Procmail.
Any
program that actually handles a message for delivery to the point where it can
be read by an email client application can be considered an MDA. For this
reason, some MTAs (such as Sendmail and Postfix) can fill the role of an MDA
when they append new email messages to a local user’s mail spool file. In
general, MDAs do not transport messages between systems nor do they provide a
user interface; MDAs distribute and sort messages on the local machine for an
email client application to access.
Mail User
Agent(MUA)
A
Mail User Agent (MUA) is synonymous with an email client application. An MUA is
a program that, at a minimum, allows a user to read and compose email messages.
Many MUAs are capable of retrieving messages via the POP or IMAPprotocols,
setting up mailboxes to store messages, and sending outbound messages to an
MTA.
MUAs
may be graphical, such as Evolution,Thunderbird & Outlook, or have simple
text-based interfaces, such as Mutt
