As we outlined at the beginning of the previous chapter, the actual movement of e-mail between systems is accomplished through SMTP: the "Simple Mail Transport Protocol." It was first defined in 1982 in RFC 821; the most recent RFC defining it is 5321. It typically serves in two roles:
When a user types an e-mail message on a laptop or desktop machine, the e-mail client uses SMTP to submit the e-mail to a real server that can send it along to its destination.
E-mail servers themselves use SMTP to deliver messages, sending them across the Internet to the server in charge of the recipient e-mail address's domain (the part of the e-mail address after the
There are several differences between how SMTP is used for submission ...