Mail delivery is arguably one of the most important functions of the Internet today. Linux provides a plethora of options in mail servers, including the old standard, sendmail, and newer upstart servers like Exim and Postfix. No matter what SMTP server you choose to use, you can configure it to handle various common functions, including accepting mail addressed to it and relaying mail to other systems. You must attend to many details, such as the range of hosts for which a server will relay mail and any anti-spam configuration you may want to implement. After an SMTP server receives mail, it often relies upon another utility, Procmail, to further process the mail. Procmail may perform spam checks, run programs in response to mail that meets certain criteria, forward or copy mail to others, and so on. Both the system administrator and individual users may configure Procmail, so it's an extremely flexible tool.