Three kinds of mail services are available: Message Transfer Agents (MTA), Mail Delivery Agents (MDA), and Mail User Agents (MUA).
An MTA is a server that sends e-mail through a network. Linux uses MTA agents such as sendmail, which uses the SMTP protocol to send e-mail over a TCP/IP network like the Internet.
An MDA is a mail processor. It takes messages from the Internet and stores them in servers or spools where mail readers (a.k.a MUAs) ”such as pine , Mozilla Mail, KMail, and Evolution ”can read them. The most common example of an MDA is procmail. While the procmail-* RPM is installed by default, it works seamlessly with a properly configured sendmail (and other outgoing e-mail server) package.
An MUA is an application that helps you send and receive e-mail through these servers. Most users are familiar with at least one MUA, like those listed, or Lotus Notes, Netscape, or pine . When you prepare and send an e-mail message, you re using an MUA to send a message to an MTA such as sendmail.
The two major protocols for receiving e-mail are POP3 and IMAP4. Mail servers configured to either protocol are simply one step in the MDA process.
All you need to do to configure POP3 or IMAP4 is enable their respective configuration files ( pop3s and imaps ) in the /etc/xinted.d directory. See Chapter 23 for more information.
A substantial number of TCP/IP protocols are involved in sending an e-mail message from one user to another. We ve mentioned three of them: SMTP, POP3, and IMAP4.
As a Linux administrator, you ve probably set up an e-mail server at some point in time. While sendmail is the most important of the SMTP servers, several alternatives are available, including Exim, Postfix, and Qmail.
As a Linux administrator, you may also help users configure their e-mail clients . Generally, you ll need to know the names of any incoming e-mail servers on your network or with your ISP. This may include the name of the mail exchanger ( MX ) record that you created in your DNS server in Chapter 24 . But this chapter is focused on outgoing mail.
Older mail servers used the Unix-to-Unix Copy Protocol (UUCP), which sent messages directly from computer to computer. If the message had to go to a different network, you would have to specify each computer on the path . Needless to say, this has become unwieldy with the expansion of the Internet.
While the rest of this chapter is focused on sendmail, there are alternatives, based on the search for the easy-to-configure e-mail server. Packages for each of these systems (except the commercial version of sendmail, which is S endmail) are available from sources such as www. rpmfind .net .
Commercial Sendmail Unlike the version included with Red Hat Linux, the commercial version of Sendmail is designed for the enterprise. In other words, it can help you serve many thousands of users. It is even configurable for mobile clients. More information is available at www.sendmail.com .
Exim The Exim MTA was developed at Cambridge (U.K.) and is licensed under the GPL. While based on an older MTA known as Smail, it can also help you verify user addresses and refuse e-mail. This helps you minimize spam sent to users on your system. More information is available at www.exim.org .
Postfix The Postfix MTA is an alternative to sendmail that is probably already installed on your Red Hat Linux system. It is the successor to the VMailer and IBM Secure Mailer systems. As described in Chapter 19 , you can use redhat-switchmail to switch between these servers. You can find more information at www.postfix.org .
Qmail The Qmail MTA is another alternative to sendmail. According to www.qmail.org , Qmail is used by an impressive list of Internet sites. The developer, D. J. Bernstein, has offered a cash reward for the first person to find a security hole in this system ( cr.yp.to/qmail/guarantee.html ).
Smail The Smail MTA is reportedly easier to configure than sendmail. It also includes support for blocking messages. In addition, it helps you protect yourself from spoofed messages that try to mask themselves as coming from trusted sites. While no official website exists for this MTA, the developers can be found at www.planix.com .