SMTP
Simple Mail Transfer Protocol
Content
I. What is SMTP?
II. History of SMTP
III. General Features
IV. SMTP Commands
V. SMTP Replies
VI. A typical SMTP Session
VII. The Mail Object
VIII.Relaying
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.
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.
History of SMTP
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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
General Features
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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.
SMTP Commands
SMTP Replies
A typical SMTP Session
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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
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@remulaner.net
Receiver: 250 Ok
Sender: RCPT TO: riker@starfleet.org
Receiver 250 Ok
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?
Sender:<CRLF>.<CRLF>
Receiver: 250 Ok
Sender: QUIT
Receiver: 221 starfleet.org closing transmission channel
The Mail Object
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SMTP transports a mail object
Consists of envelope and content
Envelope:
Originator Address (MAIL FROM: xx@xx.net)
 Recipient Address(es) (RCPT TO: x@x.net)
 Extensions
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Content:
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Header and Body (DATA)
Relaying
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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 ??