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Title:

SMTP Simple Mail Transfer Protocol

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A typical SMTP Session. The Mail Object. Relaying. What is SMTP? ... Responsibility for the message is handed over to relay host: Deliver Message ... – PowerPoint PPT presentation

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Title: SMTP Simple Mail Transfer Protocol


1
SMTPSimple Mail Transfer Protocol
2
Content
  1. What is SMTP?
  2. History of SMTP
  3. General Features
  4. SMTP Commands
  5. SMTP Replies
  6. A typical SMTP Session
  7. The Mail Object
  8. Relaying

3
What is SMTP?
  • The Simple Mail Transfer Protocol (SMTP) is the
    actual protocol used to transfer mail from MTA to
    MTA.
  • MTA Mail Transfer Agent .... A program that
    actually routes and delivers mail.
  • MUA Mail User Agent ... A program that is used
    to access mail after delivery.

4
What is SMTP?
  • The objective of SMTP is to transfer mail
    reliably and efficiently.
  • SMTP is widely deployed and proven to be very
    robust.
  • The SMTP model is based on what is called a mail
    transaction. Sender and receiver MTAs send
    commands and replies in a structured, lock-step
    process.

5
History of SMTP
  • The SMTP specification originally started with
    the Mail Transfer Protocol in 1980
  • It evolved into Simple Mail Transfer Protocol
    (SMTP) in 1981
  • Specified in RFC 821
  • Most recent RFC is 2821
  • First and most important implementation is
    sendmail

6
General Features
  • Independent of underlying transmission subsystem
  • Just needs a reliable ordered data stream channel
  • Operates over TCP port 25
  • Uses an request-response protocol that is useful
    to human readers (SMTP commands)
  • SMTP sessions are stateful, with both parties
    carefully maintaining a common view of the
    current state.

7
SMTP Commands
8
SMTP Replies
9
A typical SMTP Session
  • SMTP commands are generated by the SMTP client
    and sent to the SMTP server.
  • SMTP replies are sent from the SMTP server to the
    SMTP client in response to the commands.
  • To effect a mail transfer to an SMTP server, an
    SMTP client establishes a two-way transmission
    channel to that SMTP server.
  • Client resolves either final target host or
    intermediate MX host via DNS MX RRs

10
A typical SMTP Session
  • Sender Open TCP connection to port 25
  • Receiver 220 starfleet.org ready
  • Sender HELO remulaner.net
  • Receiver 250 starfleet.org
  • Sender MAIL FROM checker_at_remulaner.net
  • Receiver 250 Ok
  • Sender RCPT TO riker_at_starfleet.org
  • Receiver 250 Ok

11
A typical SMTP Session
  • Sender DATA
  • Receiver 354 start mail input
  • Sender SUBJECT Hi Riker Du Lutscher!
  • Sender Bla blabla Bla Blaaa! Willste ein
    Spacetrip?
  • SenderltCRLFgt.ltCRLFgt
  • Receiver 250 Ok
  • Sender QUIT
  • Receiver 221 starfleet.org closing transmission
    channel

12
The Mail Object
  • SMTP transports a mail object
  • Consists of envelope and content
  • Envelope
  • Originator Address (MAIL FROM xx_at_xx.net)
  • Recipient Address(es) (RCPT TO x_at_x.net)
  • Extensions
  • Content
  • Header and Body (DATA)

13
Relaying
  • SMTP Server maybe ultimate destination or
    intermediate relay host
  • SMTP server becomes client if it sends the
    message on
  • Responsibility for the message is handed over to
    relay host
  • Deliver Message
  • Or report failure to do so!

14
  • Thank you for listening ?
  • Any questions ??
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