Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

SMTP is a simple protocol that controls the exchange of e-mail messages between two mail servers. The protocol is used in the Internet and is defined by the IETF. Using SMTP, a process can transfer mail to another process on the same network or to some other network via a relay or gateway process accessible to both networks. A mail message may pass through a number of intermediate relay or gateway hosts on its path from sender to ultimate recipient. The basic SMTP model is pictured in the following illustration.

The model consists of user agents, which are applications that provide e-mail services (create and view messages). The MTA (message transfer agent) is the component that uses the SMTP protocol to forward messages to other MTAs. In the illustration, a user creates a message and chooses Send. The message is queued locally with other Outbox messages for delivery at a designated time. The local MTA forwards the messages to the relay MTA, which may be a corporate e-mail server located at the LAN/Internet connection, or a mail server at an ISP that home users connect with across their dial-up connection.

SMTP uses a client/server relationship. The client is the system with mail to send. It establishes a two-way transmission channel to an SMTP server over a TCP connection.