Essential System Administration, 3rd Edition

Configuring the Transport Agent

Setting up the transport agent is perhaps the most crucial mail-related job presented to the system administrator. There are a variety of transport agents available on Unix systems, but sendmail is by far the most widely used. According to current estimates, sendmail handles over 75% of all Internet mail traffic (Unix and non-Unix alike). Other transport agents used on Unix systems include Postfix, smail, qmail, and exim. We will consider sendmail and Postfix here.

sendmail

Eric Allman’s sendmail package is a very powerful facility, capable of handling email from the moment a user submits a message from a mailer program, transporting it across a LAN or the Internet to the proper destination system, and then finally handing it off to the delivery agent, which actually places the message in the user’s mailbox. In fact, because the package includes a delivery agent program, the facility as a whole can handle every aspect of electronic mail except composing and reading messages and retrieving them from message stores. sendmail is also a well-proven facility, and, at this point, is quite secure, provided that it is configured properly.

Tip

There are commercial and free versions of sendmail. The commercial versions, developed and sold by Sendmail, Inc., include additional features as well as easy-to-use graphical interfaces, integration with other related commercial products (e.g., virus-scanning software), and technical support. Vendor-supplied versions ...