In this chapter, we ll look at one of the essential applications for any computer that is connected to a network: e-mail. Various server services are used to send and to receive e-mail, and each server is associated with one or more protocols. While e-mail clients are relatively straightforward to configure, e-mail servers have a number of rich and complex options.
There are several basic TCP/IP protocols related to e-mail. The two most common protocols for receiving e-mail are the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP). The Simple Mail Transfer Protocol (SMTP) is an important protocol for sending mail from your network.
The most common SMTP e-mail server on the Internet is sendmail. While the basic sendmail configuration file is complex, Red Hat Linux includes a macro file that is easy to customize based on what you need. You can edit this file and then use a macro processor to generate a custom configuration file for sendmail. This configuration file also helps you address security needs by setting up responses for FQDN that you can t verify and domains where you don t want to send mail.
There are two basic servers for incoming e-mail, based on the IMAP4 and POP3 protocols. You can create your own incoming e-mail server, or you can set up your e-mail clients to use incoming e-mail servers from an outside e-mail provider. Red Hat Linux includes both of these servers, in regular and secure versions, in one RPM package.
Most computer users are familiar with at least one e-mail client. The principles behind them are the same. They take the e-mail data and format it in a fashion to which you can easily read and reply. Linux includes both text and graphical e-mail clients.