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MTA or Message Transfer Agent
the exchange of email using TCP/IP defines how
email messages are transmitted between two users.

Next Page - 2 Basically, email consist of:
(1) The envelope  (2) headers   (3) Message body

This Page 1 Exchange of email using TCP/IP


The exchange of email
TCP/IP (Transmission Control Protocol / Internet Protocol) is performed by MTA (Message Transfer Agent) that defines how email messages are transmitted between two users and MTA defines how email messages are transmitted between two users.

the sending of email consist of 2 parts
User Agent and MTA


that prepares and send emails, explained in rfc 821 for the basic specification of SMTP

SMTP is the guide lines for basic specification of service extension and authentication message submission of Internet host requirements delivery status and notifications on message submission for mailings service extension for returning enhanced error codes. See Basic Email Delivery

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SMTP Simple Mail Transport Protocol a protocol governing email transmission and reception (sending and receiving emails between mail servers). See: How Email Works

User Agent
Any program like: Eudora, MS Outlook, Gmail, Yahoo-mail or editor that let users create, edit, compose, store, send mails to a mail server
, polls a mail server for mails, download and display mails and use POP3 to talk to Mail server.


A typical example of how two MTAs communicate with each other using a single TCP connection.

Below "The session" describing a one-line email message sent by using SMTP Simple Mail Transport Protocol governing email transmission and reception?

The MTA used for this session is Sendmail--a software program for sending emails.

-V MTA sent/received across the SMTP connection 220 a 3 digit numeric reply code from the SMTP server

>>> commands sent by the SMTP client

The session...

sun % mail -V
To: (output by sender user agent)
Subject: testing (sender prompted for a subject)
user agent adds a blank line between headers and body
8, 9, 10.
. a period to end the message

Sending Letter...mable@test@org output by user agent followed by output from MTA (Sendmail)
Connecting to mailhost via ether...
Trying connected

220 Sendmail 4.0/SAG-test.G89 ready at Tue, 21 May 98 11:37:24 MST

>>> HELO
250 Hello, pleased to meet you

>>> MAIL From:<>
250 <>... Sender ok

>>> RCPT To:<>
250 <>... Recipient ok

>>> DATA
354 Enter mail, end with "." on a line by itself

250 Mail accepted

>>> QUIT
221 delivering mail Sent (output by user agent)

A one-line message data  8, 9, 10. was to the User Agent
but 12 lines went to the client. The first three lines between Received: and Message-Id: are added by the MTA and the rest were generated by the User Agent

Received: by (4.1/SMI-4.1)
id AA01301; Tue, 21 May 98 11:37:22 MST
Message-Id: <>
From: (Mable Merliney)
X-Phone: +0 202 671 2761
X-Mailer: Mail Userís Shell (9.1.0 12/02/98)
Subject: testing

8, 9, 10.

  SMTP Commands

Sender type mail to invoke sender user agent.

Sender prompted for a subject: testing
user agent adds a blank line between headers and body
Sender type message body 8, 9, 10.
Sender type a period
. to completes the message and...

User Agent passes the mail to the MTA for delivery.
The client opens to TCP port 25
Connecting to mailhost via ether...
Trying connected

waits for a greeting message, reply code 220 from the server and response to start with a qualified domain name of the serverís host:

>>> HELO Command, the client identifies itself
The argument must be a qualified domain name of the client host:

>>> MAIL From: Command identifies the message

>>> RCPT To: Command, identifies the recipient.
If there are multiple recipients, more than one RCPT command can be issued.
>>> DATA Command, the client use it to send the
                                 contents of the mail message.
Command is a request for mail service action sent by the sender SMTP to the receiver SMTP.

>>> QUIT Command, terminates mail exchange.

QUIT command, the receiving machine must send an OK reply to the sending machine--at this point--the receiving machine do not close the transmission yet--until it receives and reply to the QUIT command from the sender machine. The sender machine will also not close the transmission until it sends the QUIT command and receives a reply.

If the connection is closed prematurely, a 504 command not implemented failure reply to the sending machine and the receiving machine will act as if a RSET command had been received, cancel pending transaction due to temporary error. A RSET command specifies that current mail transaction is to be aborted and the receiver machine must send an OK reply.

RSET command aborts current mail transaction and causes both ends to reset and any stored information about sender, recipients or mail data is discarded. A mail transaction may be aborted by the RSET command.

If the transaction beginning command argument is not acceptable a 501 failure reply must be returned. If the commands in a transaction are out of order a 503 failure reply must be returned. See SMTP Syntax error codes

Continue... Next Page - 2
Basically, email consist of:
(1) The envelope
(2) headers
(3) Message body