Chapter Summary

The sendmail daemon is an MTA (Mail Transfer Agent). When you send a message, sendmail works with other software to get it to the proper recipients. You can set up sendmail to relay email to an SMTP server that sends the email on to its ultimate destination or you can have sendmail send email directly to the SMTP servers for the domains receiving the email. By default, sendmail stores incoming messages in the mail spool directory, /var/spool/mail.

The file that controls many aspects of how sendmail works is sendmail.cf. If you edit sendmail.mc, when you restart sendmail, the sendmail init script generates sendmail.cf. The system administrator can use the /etc/aliases file and ordinary users can use ~/.forward files to reroute email to one or more local or remote addresses, to files, or as input to programs.

You can use a program such as SpamAssassin to grade and mark email as to the likelihood of it being spam. You can then decide what to do with the marked email: You can look at each piece of potential spam and decide where to put it, or you can have your MUA automatically put potential spam in a special mailbox for spam.

Other programs that can help with email include SquirrelMail, which provides Webmail services, and Mailman, which provides mailing list support.