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Lesson 4 Linux MTAs and MDAs
ObjectiveDescribe the purpose of Linux MTAs and MDAs.

Linux MTAs | MDAs

Linux has several common MTAs (Mail Transfer Agents) and MDAs (Mail Delivery Agents), described in the following table:
Program Classification Description
fetchmail MTA fetchmail transfers email from remote POP and IMAP servers to your local SMTP server. You will need fetchmail if, for example, you need to retrieve remote email, but can't use your machine as an email exchanger.
sendmail MTA and MDAsendmail, which delivers most of the Internet's mail, operates as both an MTA and a MDA. It sends email to and receives email from Internet hosts, and it delivers mail to domain mail handlers.
procmail MDA procmail is a powerful MDA that provides extremely sophisticated support for delivering mail. It allows email to be filtered and tagged before delivery, optionally filing mail into content-specific mail folders.

Selecting mail and printing alternatives

Beginning with Red Hat Linux 7.3, which was the first version to offer the alternatives feature, two major services were configured to use alternatives: mail transport and printing services. The alternatives facility let system administrators choose the following, related to mail transport and printing:
  1. Mail Transport Agent (MTA) : If the sendmail, exim and postfix mail transport agents are installed, as an administrator you can choose which of those services is the default for sending and receiving e-mail.
  2. Printing: If both LPRng and CUPS printing services are installed, you can choose which service is the default for printing documents.
As an administrator, you still need to configure each alternative service to work. Descriptions for configuring sendmail and postfix mail-transport agents are contained in Chapter 19. Information on setting up the CUPS printing service is in Chapter 17. (LPRng is no longer delivered with Fedora or RHEL, although it is still available from sites such as rpmfind.net. Likewise, the feature for switching the printing service described below is not included with Fedora or RHEL, but can still be found in earlier Red Hat Linux systems.)
In terms of setting up the alternatives side of mail services, much of the work of creating links so that the services can be chosen has already been done. Links relating to the default services are set up in the /etc/alternatives directory. Definitions that identify the alternative components of sendmail, exim, and postfix mail servers are contained in the /var/lib/alternatives directory.
Because much of the configuration has been done in advance, the first step in switching between the different mail services installed on your computer is only a couple of clicks away.
The next lesson compares several different Linux MUAs.