Notes for Simple Mail Transfer Protocol (SMTP)

Simple Mail Transfer Protocol (SMTP)

·         The actual mail transfer is done through message transfer agents (MTAs).

·         To send mail,a system must have the client MTA, and to receive mail, a system must have a server MTA.

·         The formal protocol that defines the MTA client and server in the Internet is called Simple Mail Transfer Protocol (SMTP).

·         Following fig. Shows the scenario of SMTP protocol.

·         SMTP is used two times, between the sender and the sender’s mail server and

     between the two mail servers.

·         Commands and Responses

·         SMTP uses commands and responses to transfer messages between an MTA client and

an MTA server as shown in following fig.

·         Each command or reply is terminated by a two-character end-of-line token.

·         Commands

·         Commands are sent from the client to the server. The format of a command is shown below:

·         Keyword: argument(s)

·         It consists of a keyword followed by zero or more arguments. SMTP defines 14

commands listed in following Table and described in more detail below.

·         HELO. This command is used by the client to identify itself. The argument is the

domain name of the client host. The format is HELO: challenger.atc.fhda.edu

·         MAIL FROM. This command is used by the client to identify the sender of the

message. The argument is the e-mail address of the sender (local part plus the

domain name). The format isMAIL FROM: forouzan@challenger.atc.fhda.edu

·         RCPT TO. This command is used by the client to identify the intended recipient of

the message. The argument is the e-mail address of the recipient. If there are multiple

recipients, the command is repeated. The format isRCPT TO: betsy@mcgraw-hill.com

·         DATA. This command is used to send the actual message. All lines that follow the

DATA command are treated as the mail message. The message is terminated by a

line containing just one period. The format isDATA

This is the message

to be sent to the McGraw-Hill

Company.

·         QUIT. This command terminates the message. The format is QUIT

·         RSET. This command aborts the current mail transaction. The stored information

about the sender and recipient is deleted. The connection will be reset. RSET

·         VRFY. This command is used to verify the address of the recipient, which is sent

as the argument. The sender can ask the receiver to confirm that a name identifies a

valid recipient. Its format isVRFY: betsy@mcgraw-hill.com

·         NOOP. This command is used by the client to check the status of the recipient. It

requires an answer from the recipient. Its format isNOOP

·         TURN. This command lets the sender and the recipient switch positions, whereby

the sender becomes the recipient and vice versa. However, most SMTP implementations

today do not support this feature. The format isTURN

·         EXPN. This command asks the receiving host to expand the mailing list sent as the

arguments and to return the mailbox addresses of the recipients that comprise the

list. The format isEXPN: x y z

·         HELP. This command asks the recipient to send information about the command

sent as the argument. The format isHELP: mail

·         SEND FROM. This command specifies that the mail is to be delivered to the

terminal of the recipient, and not the mailbox. If the recipient is not logged in,

the mail is bounced back. The argument is the address of the sender. The

format isSEND FROM: forouzan@fhda.atc.edu

·         SMOL FROM. This command specifies that the mail is to be delivered to the terminal

or the mailbox of the recipient. This means that if the recipient is logged in, the mail is         delivered only to the terminal. If the recipient is not logged in, the mail is

delivered to the mailbox. The argument is the address of the sender. The format is

·         SMOL FROM: forouzan@fhda.atc.edu

·         SMAL FROM. This command specifies that the mail is to be delivered to the terminal

and the mailbox of the recipient. This means that if the recipient is logged in,

the mail is delivered to the terminal and the mailbox. If the recipient is not logged

in, the mail is delivered only to the mailbox. The argument is the address of the

sender. The format is SMAL FROM: forouzan@fhda.atc.edu

·         Responses

Responses are sent from the server to the client. A response is a three-digit code that

may be followed by additional textual information.

Following Table  lists some of the responses.

·         Mail Transfer Phases

·         The process of transferring a mail message occurs in three phases: connection establishment,mail transfer, and connection termination.

i)                    Connection Establishment

After a client has made a TCP connection to the well-known port 25, the SMTP server

starts the connection phase. This phase involves the following three steps, which are

illustrated in Fig.

1.  The server sends code 220 (service ready) to tell the client that it is ready to receive

mail. If the server is not ready, it sends code 421 (service not available).

2.  The client sends the HELO message to identify itself using its domain name

address. This step is necessary to inform the server of the domain name of the

client. Remember that during TCP connection establishment, the sender and

receiver know each other through their IP addresses.

3.  The server responds with code 250 (request command completed) or some other

code depending on the situation.

i)                    Message Transfer

After connection has been established between the SMTP client and server, a single

message between a sender and one or more recipients can be exchanged. This phase

involves eight steps. Steps 3 and 4 are repeated if there is more than one recipient (see

in Fig.).

1.  The client sends the MAIL FROM message to introduce the sender of the message.

It includes the mail address of the sender (mailbox and the domain name). This

step is needed to give the server the return mail address for returning errors and

reporting messages.

2.  The server responds with code 250 or some other appropriate code.

3.  The client sends the RCPT TO (recipient) message, which includes the mail

address of the recipient.

4.  The server responds with code 250 or some other appropriate code.

5.  The client sends the DATA message to initialize the message transfer.

6.  The server responds with code 354 (start mail input) or some other appropriate

message.

7.  The client sends the contents of the message in consecutive lines. Each line is terminated

by a two-character end-of-line token (carriage return and line feed). The

message is terminated by a line containing just one period.

8.  The server responds with code 250 (OK) or some other appropriate code.

i)                    Connection Termination

1 After the message is transferred successfully, the client terminates the connection. This

phase involves two steps (see Fig).

2  The client sends the QUIT command.

3 The server responds with code 221 or some other appropriate code.

After the connection termination phase, the TCP connection must be closed.