Simple Mail Transfer Protocol (SMTP) is an Internet
standard for e-mail transmission across Internet Protocol (IP) networks. Is the
protocol in widespread use today. While electronic mail servers and other mail
transfer agents use SMTP to send and receive mail messages, user-level client
mail applications typically only use SMTP for sending messages to a mail server
for relaying.
For receiving messages, client applications usually use either the
Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) to
access their mail box accounts on a mail server. Protocol overview SMTP is a
relatively simple, text-based protocol, in which a mail sender communicates with
a mail receiver by issuing simple command strings and supplying necessary data
over a reliable ordered data stream channel, typically a Transmission Control
Protocol (TCP) connection. An SMTP session consists of a series of commands,
initiated by the SMTP client, and responses from the SMTP server through which
the session is opened, operating parameters are exchanged, the recipients are
specified, and possibly verified, and the message is transmitted, before the
session is closed.
The originating host is either an end-user' s email client
also known as mail user agent (MUA), or a relay server' s mail transfer agent
(MTA). SMTP is a delivery protocol only. It cannot pull messages from a remote
server on demand. Other protocols, such as the Post Office Protocol (POP) and
the Internet Message Access Protocol (IMAP) are specifically designed for
retrieving messages and managing mail boxes. However, the SMTP protocol has a
feature to initiate mail queue processing on a remote server so that the
requesting system may receive any messages destined for it. POP and IMAP are
preferred protocols when a user' s personal computer is only intermittently
powered up, or Internet connectivity is only transient and hosts cannot receive
message during off-line periods.
Source: Wikipedia, the free encyclopedia