The Simple Mail Transfer Protocol (SMTP) is the de facto standard for mail transport across the Internet. Anyone who wants to have a mail server capable of sending and receiving mail across the Internet must be able to support SMTP. As a standards-based protocol (RFC 5321), SMTP is well understood, platform independent, and well supported across a variety of operating systems and devices.
In this chapter, we’ll discuss the mechanics of SMTP as a protocol and its relationship to other mail-related protocols, such as Post Office Protocol (POP) and Internet Message Access Protocol (IMAP). Then we will go over the Postfix SMTP server, one of the easier and more secure SMTP servers out there.