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Simple Mail Transfer Protocol

SMTP

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What is SMTP?y It is the standard protocol for transferring e-mail between

hosts in the TCP/IP suite

y It is defined in RFC 821

y It is a relatively simple, text-based protocol

y SMTP standardized the message character set as 7-bit

ASCII

y SMTP uses TCP port 25.

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What is mail?

y Mail is a text file

Envelope -

y sender address

y receiver address

Message y Mail Header defines

the sender, the receiver,the subject of themessage, and someother information

y Mail Body Containsthe actual informationin the message

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How messages are sent to SMTP

server?

E-mail communication using Relaying

y Used during initial days of SMTP.y SMTP routing information is included along with E-mail address.y Problem with this method.

Using DNS

y This method is used at present.y The senders SMTP server makes the use of DNS to find MX record of the

domain to which the E-mail is to be sent.

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E-MAIL COMMUNICATION USING RELAYING

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SMTP architecture

y SMTP clients andservers have two maincomponents

y User Agents Prepares the

message, encloses it in an envelope.(Eudora, Outlook express etc forexample)

y Mail Transfer Agent (MTA) Transfers the mail across the

internet

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SMTP PROCEDURE

y Client·s SMTP mail server establishes a TCP

connection to the recipients SMTP server using Port 25

y three phases in message transfery Connection establishment

y Transfer of messages

y Connection termination

y command/response interactiony commands: ASCII text

y response: status code and phrase

y messages must be in 7-bit ASCII

7

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Commands of SMTP

HELOHELO : Request to initiate SMTP session: Request to initiate SMTP session

MAIL FROM : Sender¶s EMAIL FROM : Sender¶s E--Mail addressMail address

RCPT TORCPT TO :: Receiver¶s EReceiver¶s E--Mail addressMail address

DATADATA :: Body of messageBody of message

QUITQUIT :: Terminates SMTP connectionTerminates SMTP connection

RSETRSET :: Aborts mail transactionAborts mail transaction

VRFYVRFY :: Asks receiver to verify the validity of the mailboxAsks receiver to verify the validity of the mailbox

EXPNEXPN :: Asks receiver to identify mailing listAsks receiver to identify mailing list

HELPHELP :: Causes receiver to send help informationCauses receiver to send help information

NOOP NOOP :: Forces server to verify the communication with SMTPForces server to verify the communication with SMTP

receiver receiver

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Status Codesy 211 System status, or system help reply .

y 214 Help message.

y 220 <domain> Service ready.

y 221 <domain> Service closing transmission channel.

y250 Requested mail action okay, completed.

y 251 User not local; will forward to <forward-path>.

y 354 Start mail input; end with <CRLF>.<CRLF>.

y 421 <domain> Service not available, closing transmission channel.[This may be a reply to any command if the service knows itmust shut down].

y 450 Requested mail action not taken: mailbox unavailable.y 451 Requested action aborted: local error in processing

y 452 Requested action not taken: insufficient system storage.

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Status Codes

y 500 Syntax error, command unrecognized. [This may include errorssuch as command line too long]

y 501 Syntax error in parameters or arguments.

y 502 Command not implemented.

y 50

3 Bad sequence of commands.y 504 Command parameter not implemented.

y 550 Requested action not taken: mailbox unavailable.

y 551 User not local; please try <forward-path>.

y 552 Requested mail action aborted: exceeded storage allocation.

y 553 Requested action not taken: mailbox name not allowed.

[E.g., mailbox syntax incorrect]y 554 Transaction failed.

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Connection EstablishmentTCP Connection

Establishment

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Message Progress

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Connection Termination

TCP Connection Termination

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