Protocal overview


SMTP is a connection oriented, text-based protocol in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel, typically a Transmission Control Portocal (TCP) connection. An SMTP transaction consists of three command/reply sequences:
1.     MAIL command, to establish the return address, also called return-path, reverse-path, or envelope sender.
2.     RCPT command, to establish a recipient of the message. This command can be issued multiple times, one for each recipient. These addresses are also part of the envelope.
3.     DATA to signal the beginning of the message text; the content of the message, as opposed to its envelope. It consists of a message header and a message body separated by an empty line.
Besides the intermediate reply for DATA, each server's reply can be either positive (2xx reply codes) or negative. Negative replies can be permanent (5xx codes) or transient (4xx codes).
A reject is a permanent failure and the client should send a bounce message to the server it received it from. A drop is a positive response followed by message discard rather than delivery.
The initiating host, the SMTP client, can be either an end-user's email client, functionally identified as a mail user agent (MUA), or a relay server's mail transfer agent (MTA), that is an SMTP server acting as an SMTP client, in the relevant session, in order to relay mail.
Fully capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures.
A MUA knows the outgoing mail SMTP server from its configuration. A relay server typically determines which server to connect to by looking up the MX (Mail exchange) DNS resource record for each recipient's domain name.
Relay servers can also be configured to use a smart host. A relay server initiates a TCP connection to the server on the “well-known port” for SMTP: port 25, or for connecting to an MSA, port 587. The main difference between an MTA and an MSA is that connecting to an MSA requires SMTP Authentication.

SMTP vs mail retrieval

SMTP is a delivery protocol only. In normal use, mail is "pushed" to a destination mail server (or next-hop mail server) as it arrives. Mail is routed based on the destination server, not the individual user(s) to which it is addressed. Other protocols, such as the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) are specifically designed for use by individual users retrieving messages and managing mail boxes.
To permit an intermittently-connected mail server to pull messages from a remote server on demand, SMTP has a feature to initiate mail queue processing on a remote server (see Remote Message Queue Starting below).
 POP and IMAP are unsuitable protocols for relaying mail by intermittently-connected machines; they are designed to operate after final delivery, when information critical to the correct operation of mail relay (the "mail envelope") has been removed.

Remote Message Queue Starting

Remote Message Queue Starting is a feature of SMTP that permits a remote host to start processing of the mail queue on a server so it may receive messages destined to it by sending the TURN command.
This feature however was deemed insecure[17] and was extended in RFC 1985 with the ETRN command which operates more securely using an authentication method based on Domain Name System information.

On-Demand Mail Relay

On-Demand Mail Relay (ODMR) is an SMTP extension standardized in RFC 2645 that allows an intermittently-connected SMTP server to receive email queued for it when it is connected.

Internationalization

Users whose native script is not Latin based, or who use diacritic not in the ASCII character set have had difficulty with the Latin email address requirement. RFC 6531 was created to solve that problem, providing internationalization features for SMTP, the SMTPUTF8 extension and support for multi-byte and non-ASCII characters in email addresses, such as those with diacritics and other language characters such as Greek and Chinese.
 Current support is limited, but there is strong interest in broad adoption of RFC 6531 and the related RFCs in countries like China that have a large user base where Latin (ASCII) is a foreign script.


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