Simple Mail Transfer Protocol

Overview• Introduction to SMTP• History of Smtp• General features of SMTP

• Sendmail• Relays And Gate ways• How SMTP works• Limitations in SMTP• Example of SMTP

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Introduction

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Composes, reads,replies to,forwards and handles mailboxes

Transfers mails from one host to

another

Collects mail and delivers to

user agent

Transfers mails from one host to

another

Collects mail and delivers to

user agentIntroduction…

Introduction to SMTP

• SMTP is an Internet standard for electronic mail.• SMTP is the protocol in widespread use today.• It 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 Agen t ... A program that is used

to access mail after delivery.• The objective of SMTP is to transfer mail reliably

and efficiently.

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

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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SendmailMail Transfer Agent [MTA] : is a computer program or software agent that transfers electronic mail messages from one computer to another. • Sendmail is a MTA• Supports several mail transfers including SMTP• Pro’s

• Can perform header rewriting, mail routing• Extensive support available

• Con’s• Not secure• Code is bulky [compared to other MTA’s such as qmail]

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Message transfer…

SMTP is a push protocol

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• SMTP server can also assume the role of a “relay”

• SMTP mail gateways are used to transport mail prepared by a protocol other than SMTP

Relays and Gateways

SMTP supported

Non-SMTP supported

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Link Layer PCI IP-PCI

Keyword: argument(s)

Command format:

Response format:

3-digit status code [textual information]

How SMTP works?

Link Layer PCI IP-PCI TCP-PCI SMTP command/responseTCP-PCI SMTP command/responseTCP-PCI SMTP command/response

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How SMTP works : Status Codes

The Server responds with a 3 digit code that may be followed by text info– 2## - Success– 3## - Command can be accepted with

more information– 4## - Command was rejected, but error

condition is temporary– 5## - Command rejected, Bad User!

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Limitations in SMTP

• Only uses NVT 7 bit ASCII format– How to represent other data types?

• Susceptible to misuse (Spamming, faking sender address)

SMTP transport example