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TOPIC:THE WHY,WHAT AND HOW OF EMAIL
M.S RAMAIAH INSTITUTE OF
What is Electronic Mail?
• Electronic Mail - It is a facility that allows users to
transmit messages across the Internet.
• It is the most widely used application service offering
a fast conventional method of transferring information.
• It can accommodate large messages or voluminous
• It allows communication between groups or single
HOW EMAIL IS SENT
Post Office Protocol
Version 3 (POP3)
• Post Office Protocol Version 3 has been the most popular protocol
has been the most popular protocol used to transfer messages from
a permanent mailbox to a local computer.
• When the user invokes a POP3 client, it establishes a TCP
connection and contacts a POP3 server on the mailbox computer.
The user sends a login and a password to authenticate the session.
If login was successful, the client sends commands to retrieve
copies of messages and to delete messages from the permanent
• Both SMTP servers and POP3 servers must synchronize access to
• USER name: User name for authentication
• PASS password: Password used for
• STAT: Get number and total size of message
• LIST: [msg] get size of message
• RETR: msg Send message to client
• DELE: msg Delete message from mailbox
• RSET: Cancel previous delete requests.
• QUIT: Updates mailbox (deletes messages) and
Internet Message Access
• An alternative to POP3 is IMAP version 4. It defines an
abstraction known as a MAILBOX. Mailboxes are located on
the same computer as a server.
• IMAP4 is a method for accessing electronic mail messages
that are kept on a mail server. It permits a client
e-mail program to view and manipulate those messages.
• Electronic mail stored on an IMAP server can be viewed
or manipulated from a desktop computer at home,
a notebook computer, or at a workstation. We can also say
that mail messages can be accessed from multiple
What are the functions
Includes operations for:
• creating mailboxes
• deleting mailboxes
• renaming mailboxes
• checking for new messages
• permanently removing messages
• setting and clearing flags
• fetching of message attributes texts, and
• IMAP provides extended functionality for
message retrieval and processing.
• Users can obtain information about a
message or examine header fields without
retrieving the entire message.
• Users can search for a specified string and
retrieve portions of a message. This is
useful for slow-speed dialup connections
since they wont need to download useless
Introduction to SMTP
• The TCP/IP Protocol specifies a standard for the
exchange of mail between machines. This
standard specifies the exact format of messages
a client on a single machine uses to transfer
mail to a server on another.
• This standard transfer protocol is known as the
Simple Mail Transfer Protocol. (SMTP) The main
objective of SMTP is to provide the reliability and
efficiency of mail transfer.
• SMTP basically focuses on how the underlying
system passes messages across an internet
from machine to
• SMTP does not specify how the mail system
from a user or how the user interface presents
• An important feature of SMTP is "mail relaying."
Mail relaying is SMTP's capability of transporting
• Through SMTP, a process can transfer mail to
process using the same or other networks via a
process accessible to both networks.
• Mail messages can be passed through a
number of intermediate
relay or gateways hosts from sender to ultimate
Basic SMTP Design
Client SMTP Server SMTP
Simple Mail Transfer Protocol (SMTP) is an Internet standard for
electronic mail (email) transmission. Although electronic mail
servers and other mail transfer agents use SMTP to send and
receive mail messages, user-level client mail applications
typically use SMTP only for sending messages to a mail server
for relaying. For receiving messages, client applications usually
use either POP3 or IMAP.
A message transfer agent receives mail from either another MTA,
a mail submission agent (MSA), or a mail user agent (MUA).
The transmission details are specified by the Simple Mail
Transfer Protocol (SMTP). When a recipient mailbox of a
message is not hosted locally, the message is relayed, that is,
forwarded to another MTA.
Every time an MTA receives an email message, it adds
a received trace header field to the top of the header of the
message,thereby building a sequential record of MTAs handling
the message. The process of choosing a target MTA for the next
hop is also described in SMTP, but can usually be overridden by
configuring the MTA software with specific route
Mail security and spoofing
• SMTP is not a secure protocol.
• A user may spoof, which means they may trick the
protocol to send a message posing as someone else.
• Usually a user will prove that they are authentic in the
message body with a digital signature or SPF (sender
policy framework) records.
• SPF records work by telling people which machines you
send email from, and if the sender is not one of those
machines then they are lying.
• If relay is on, no authentication possible.
• Spam was a huge problem early on because early
defaults had relay on and people didn't know how to turn
Scope of operation of SMTP
• An SMTP server may refuse to accept mail for any operational or
technical reason that makes sense to the site providing the server.
• If sites take excessive advantage of the right to reject traffic, the
ubiquity of email availability (one of the strengths of the Internet) will
• Considerable care should be taken and balance maintained if a site
decides to be selective about the traffic it will accept and process.
• In recent years, use of the relay function through arbitrary sites has
been used as part of hostile efforts to hide the actual origins of mail.
• Some sites have decided to limit the use of the relay function to
known or identifiable sources, and implementations should provide
the capability to perform this type of filtering.
• When mail is rejected for these or other policy reasons, a 550 code
should be used in response to EHLO, MAIL, or RCPT as