SMTP Special Features, Capabilities, and Extensions
The primary job of the SMTP is to implement the TCP/IP email delivery system. Whenever the user of an SMTP server gives it an email message addressed to a remote mailbox, the server will attempt to transfer it to the appropriate destination server, using the SMTP mail transaction process. Many billions of such transfers are performed every day on the Internet, allowing email to reach its destination quickly anywhere around the world.
SMTP Special Features and Capabilities
In addition to this basic transfer mechanism, SMTP includes a number of other features and capabilities. These allow SMTP to support special requirements and auxiliary needs of the mail system, as described in detail in RFC 2821. ...