Electronic mail is still the most important user service on the network. The Web carries a greater volume of traffic, but e-mail is the service used for most person-to-person communication. And person-to-person communication is the real foundation of business. No network is complete without e-mail, and no network server operating system is worth its salt if it doesn't include full TCP/IP mail support.

Simple Mail Transport Protocol (SMTP) is the TCP/IP mail transport protocol. Linux provides full SMTP support through the sendmail program, although sendmail does more than just send and receive SMTP mail. sendmail provides mail aliases and acts as a "mail router," routing mail from all of the different user mail programs to the various mail delivery programs while ensuring that the mail is properly formatted for delivery.

This chapter looks at your role in configuring each of these functions. Configuring sendmail can be a large and complex task, but it doesn't have to be. Compared to some network server systems that require a second installation just to install the SMTP server software, Linux distributions do a lot of the configuration for you, and for most sites, the default configuration works fine. This chapter will give you the information you need to make intelligent decisions about when and how to change the default configuration.

sendmail configurations are built using the m4 macro processing language. The output of the m4 process is the sendmail.cf file, which is the configuration file read by sendmail. To fully understand and manage sendmail, you need to understand its functions, the sendmail.cf file from which it reads its configuration, and the m4 macros used to build that file. This chapter covers all three topics.

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