1. Introduction
The objective of the Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently.
SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream channel. While this document specifically discusses transport over TCP, other transports are possible. Appendices to RFC 821std10(-> 2821prop) describe some of them.
An important feature of SMTP is its capability to transport mail across networks, usually referred to as "SMTP mail relaying" (see section 3.8). A network consists of the mutually-TCP-accessible hosts on the public Internet, the mutually-TCP-accessible hosts on a firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN environment utilizing a non-TCP transport-level protocol. 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.
In this way, a mail message may pass through a number of intermediate relay or gateway hosts on its path from sender to ultimate recipient. The Mail eXchanger mechanisms of the domain name system [22, 27] (and section 5 of this document) are used to identify the appropriate next-hop destination for a message being transported.