What is Simple Mail Transfer Protocol (SMTP)
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. Please note that an important feature of SMTP is its capability to transport mail
across networks, usually referred to as “SMTP mail relaying”.
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.
By 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] are used to identify the appropriate next-hop destination for a message being transported.
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