FreeBSD® Unleashed

Introduction to SMTP

The Simple Mail Transfer Protocol (SMTP) is one of the oldest protocols on the Internet, nearly as old as e-mail itself, which began with Ray Tomlinson in 1971 (only two years after the first ARPA network was installed). SMTP's purpose is to transfer plain-text messages from one host to another—often in sequence, as when a dial-up client running Windows sends an e-mail message to the ISP's mail server, which then turns around and transfers it to the recipient's mail server. This sequential nature doesn't need to get much longer than this, though; it isn't like the Internet itself, with its router-to-router hopping of individual packets. The sender's SMTP server communicates directly with the recipient's SMTP server.

SMTP ...