FreeBSD6 Unleashed

Introduction to SMTP

The Simple Mail Transfer Protocol (SMTP) is one of the oldest communication protocols on the Internet—nearly as old as email itself. SMTP’s purpose is to transfer plain-text messages from one host to another. This transfer often occurs between several servers in sequence, as when a dial-up client running Windows sends an email message to the ISP’s mail server, which then transfers it to the recipient’s mail server to be picked up by the recipient’s mail program. SMTP sequences rarely get much longer than this example, though. It isn’t like the Internet itself, with its hierarchical routing structure in which a packet is handed off by as many as 20 or 30 routers on its journey from one end to the other. Rather, the sender’s ...