CCIE Security Exam Certification Guide

Simple Mail Transfer Protocol

The Simple Mail Transfer Protocol (SMTP) mechanism is used for providing e-mail services to IP devices over the Internet. SMTP is defined in RFC 821. Typically, two mail servers will talk SMTP to exchange mails. After the mails are exchanged, the users can read/retrieve their mail from the mail server. This can be done using any mail client, such as Pine, Eudore, Outlook, and so on, which use different protocols, such as Post office protocol or POP3, to connect to the server. SMTP uses well-known ports TCP port 25 and UDP port 25.

A process or daemon running on a server will use SMTP to send mail to clients. A program called Sendmail is a common tool used for SMTP mail transfer. Recently, a new release of SMTP, called ...