SMTP(Simple Mail Transfer Protocol)

SMTP(Simple Mail Transfer Protocol)

•       Simple Mail Transfer Protocol is a protocol for sending e-mail messages between servers or between a mail client and a server.

•       Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP.

•       It works on port 25.

•       SMTP is a 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 Protocol (TCP) connection.

•       An SMTP session consists of commands originated by an SMTP client (the initiating agent, sender, or transmitter) and corresponding responses from the SMTP server (the listening agent, or receiver) so that the session is opened, and session parameters are exchanged. A session may include zero or more SMTP transactions.

•       An SMTP transaction consists of three command/reply sequences. They are:

1.  MAIL command, to establish the return address, a.k.a. Return-Path, 5321.From, mfrom, or envelope sender. This is the address for bounce messages.

2. RCPT command, to establish a recipient of this message. This command can be issued multiple times, one for each recipient. These addresses are also part of the envelope.

3. DATA to send the message text. This is 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. DATA is actually a group of commands, and the server replies twice: once to the DATA command proper, to acknowledge that it is ready to receive the text, and the second time after the end-of-data sequence, to either accept or reject the entire message.

SMTP transport example

•       A typical example of sending a message via SMTP to two mailboxes (alice and theboss) located in the same mail domain (example.com) is reproduced in the following session exchange.

•       The protocol exchanges are prefixed for the server (S:) and the client (C:). After the message sender (SMTP client) establishes a reliable communications channel to the message receiver (SMTP server), the session is opened with a greeting by the server, usually containing its fully qualified domain name (FQDN), in this case smtp.example.com.

•       The client initiates its dialog by responding with a HELO command identifying itself in the command's parameter with its FQDN (or an address literal if none is available)

S: 220 smtp.example.com ESMTP

Postfix C: HELO relay.example.org

S: 250 Hello relay.example.org, I am glad to meet you C: MAIL FROM:<bob@example.org>

S: 250 Ok

C: RCPT TO:<alice@example.com>

S: 250 Ok

C: RCPT TO:<theboss@example.com>

S: 250 Ok

C: DATA

S: 354 End data with <CR><LF>.<CR><LF> C:

From: "Bob Example" <bob@example.org> C:

To: "Alice Example" <alice@example.com> C:

Cc: theboss@example.com

C: Date: Tue, 15 Jan 2008 16:02:43 -0500 C: Subject: Test message

C:

C: Hello Alice.

C: This is a test message with 5 header fields and 4 lines in the message body. C: Your friend,

C: Bob

C: .

S: 250 Ok: queued as 12345

C: QUIT

S: 221 Bye

{The server closes the connection}

•       The client notifies the receiver of the originating email address of the message in a MAIL FROM command. In this example, the email message is sent to two mailboxes on the same SMTP server: one each for each recipient listed in the To and Cc header fields.

•       The corresponding SMTP command is RCPT TO. Each successful reception and execution of a command is acknowledged by the server with a result code and response message (e.g., 250 Ok).

•       The transmission of the body of the mail message is initiated with a DATA command after which it is transmitted verbatim line by line and is terminated with an end-of-data sequence. This consists of a new-line (<CR><LF>), a single full stop (period), followed by another new-line.

Since a message body can contain a line with just a period as part of the text, the client sends two periods every time a line starts with a period; correspondingly, the server replaces every sequence of two periods at the beginning of a line with a single one. Such escaping method is called dot-stuffing.

•       The server's positive reply to the end-of-data, as exemplified, implies that the server has taken the responsibility of delivering the message. A message can be doubled if there is a communication failure at this time, e.g. due to a power shortage: Until the sender has not received that 250 reply, it must assume the message was not delivered.

•       After the receiver has decided to accept the message, it must assume the message has been delivered to it. Thus, during this time span, both agents have active copies of the message that they will try to deliver.

•       The probability that a communication failure occurs exactly at this step is directly proportional to the amount of filtering that the server performs on the message body, most often for anti-spam purposes. The limiting timeout is specified to be 10 minutes.

•       The QUIT command ends the session. If the second recipient were located elsewhere, the client would QUIT and connect to the appropriate SMTP server after the first message had been queued. The information that the client sends in the HELO and MAIL FROM commands are added (not seen in example code) as additional header fields to the message by the receiving server. It adds a Received and Return-Path header field, respectively.