SMTP Simple Mail Transfer Protocol

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SMTPSimple Mail Transfer Protocol
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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

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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.

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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.

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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

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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.

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SMTP Commands
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SMTP Replies
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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

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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

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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

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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)

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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!

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• Thank you for listening ?
• Any questions ??