Chapter 20. sendmail: Setting Up Mail Clients, Servers, and More

20. sendmail: Setting Up Mail Clients, Servers, and More

IN THIS CHAPTER

JumpStart I: Configuring sendmail on a Client	630
JumpStart II: Configuring sendmail on a Server	631
How sendmail Works	632
Configuring sendmail	635
SpamAssassin	640
Webmail	644
Mailing Lists	646
Setting Up an IMAP or POP3 Server	647
Setting Up KMail	648
Authenticated Relaying	650

Sending and receiving email require three pieces of software. At each end, there is a client, called an MUA (Mail User Agent), which is a bridge between a user and the mail system. Common MUAs are mutt, KMail, Thunderbird, and Outlook. When you send an email, the MUA hands it to an MTA (a Mail Transfer Agent such as sendmail), which transfers it to the destination server. At the destination, an MDA (a Mail Delivery Agent such as procmail) puts the mail in the recipient's mailbox file. On Linux systems, the MUA on the receiving system either reads the mailbox file or retrieves mail from a remote MUA or MTA, such as an ISP's SMTP (mail) server, using POP (Post Office Protocol) or IMAP (Internet Message Access Protocol).

Most Linux MUAs expect a local copy of sendmail to deliver outgoing email. On some systems, including those with a dialup connection to the Internet, sendmail relays email to an ISP's mail server. Because sendmail uses SMTP (Simple Mail Transfer Protocol) to deliver email, sendmail is often referred to as an SMTP server.

In the default Red Hat Linux setup, the sendmail MTA uses procmail as the local MDA. In turn, procmail writes email to the end of the recipient's mailbox file. You can also use procmail to sort email according to a set of rules, either on a per-user basis or globally. The global filtering function is useful for systemwide filtering to detect spam and for other tasks, but the per-user feature is largely superfluous on a modern system. Traditional UNIX MUAs were simple programs that could not filter mail and thus delegated this function to MDAs such as procmail. Modern MUAs, by contrast, incorporate this functionality.

Tip: You do not need to set up sendmail to send and receive email

Most MUAs can use POP or IMAP for receiving email. These protocols do not require an MTA such as sendmail. As a consequence, you do not need to install or configure sendmail (or another MTA) to receive email. You still need SMTP to send email. However, the SMTP server can be at a remote location, such as your ISP, so you do not need to concern yourself with it.