E-mail is an asynchronous communication medium. Electronic mail is used for sending a single message that includes text, voice, video or graphics to one or more recipients.
• Electronic mail is fast, easy to distribute and inexpensive.
• Simple Mail Transfer Protocol (SMTP) is the standard mechanisms for electronic mail in the internet. SMTP is the TCP/IP mail delivery protocol.
E-mail is not a real-time service in that fairly large delays can be tolerated.
Three major components are
1. User agents.
2. Mail servers.
To send e-mail to some one, the Internet e-mail address must be known to sender. E-mail addresses look like this
The e-mail address has two main parts, joined by In this example, Vilas is the username. Username can contain numbers, underscores, periods and some other special characters. Commas, spaces and parentheses are not allowed.
Hotmail.com is the host or domain name. E-mail address is case insensitive. works just the same as E-mail addresses do not have punctuation marks around them.
The message headers include the addresses of the receiver and the sender. Each header consists of the type of header, a colon, and the content of the header. Following
E-mail that supports formatting such as boldface and underlining is a recent development. In the past, e-mail consists only of text characters. If both side supports the formatted e-mail, then both sides use send/receive formatted e-mail. The formatted e-mail comes in the following type.
b) Rich text
c) Multipurpose Internet Mail Extension (MIME)
d) MS word format
HTML tags are just like web pages. It can include text formatting, numbering, bullets, horizontal lines, backgrounds, hyperlinks, and HTML styles. It uses MIME protocol for sending. Rich text can be read by most word processing applications. MIME formatting are created just for e-mail. MIME formatting can include text formatting, pictures, video, sound, and other information. MS word format uses microsoft word and all of its features as your e-mail editor.
To allow transmission of non-ASCII data through e-mail, the MIME used. MIME does not change SMTP or replace it.
is the sample of the complete header for a message.
The MIME-version declares that the message was composed using version 1.0 of the MIME protocol. The content-type declaration specifies that the data is GIF image and the content-transfer-encoding header declares that encoding was used to convert the image to ASCII. To view the image, a receiver mail system must first convert from base 64 encoding back to binary.
A content-type declaration must contain two identifiers, a content-type and a subtype, separated by a slash. In the example, image is the content type and gif is the subtype. The standard defines seven basic content types.
Functions of E-mail
E-mail system support five basic functions. They are as follows.
It is a process of creating messages and answers. When answering a message, the e-mail system can extract the originator’s address from the incoming e-mail.
It is moving messages from the originator to the receiver.
It inform the originator what happened to the message.
is the last step and related what the receiver does with the message after receiving it.
User Agent and Message Transfer Agent
E-mail system consists of two subsystems.
1. User agent
2. Message transfer agent.
1. User agent: It allow user to read and send e-mail. The user agents are local
program that provide a command based, menu based or graphical method for interacting with the e-mail system. To send an e-mail message, a user must provide the message, the destination address. The destination address should be in proper format and the user agent can deal with destination address. Details of e-mail address, we already studied in email addressing section. Most e-mail system support mailing lists, so that a user can send the same message to a list of people with a single command. For reading e-mail, the user agent will look at the user’s mail box for incoming e-mail before displaying anything on the screen. It display total number of new mail.
2. Message transfer agent: Message Transfer Agent (MTA) move the messages from
the source to the destination. MTA are system program that run in the background and move e-mail through the system. After writing the mail, user click of send icon. MTA activates at this time, MTA checks the destination address and transfer the mail to proper destination on the network. MTA use different types of protocol for moving the message from source to destination.
1. It must handle temporary failures, if a destination machine is temporarily unavailable, it must spool the message on the local machine for later delivery.
2. MTA must distinguish between local and remote destinations. 3. It may have to deliver copies of a message to several machines.
4. It may allow mixing text, voice and video in a message as well as appending documents and files to a message.
Simple Mail Transfer Protocol (SMTP)
SMTP is application layer protocol of TCP/IP model.
transfers message from sender’s mail servers to the recipients mail servers SMTP interacts with the local mail system and not the user.
SMTP uses a TCP socket on port 25 to transfer e-mail reliably from client to
Client/Server interaction follows and paradigm.
Commands are plain ASCII text.
Responses are a status code and an optional phase.
Command and response lines terminated with CRLF
Mail client application interacts with a local SMTP server to initiate the delivery
an e-mail message There is an input queue and an output queue at the interface between the local
mail system and the client and the server parts of the SMTP
The local mail system retains a mailbox for each user into which the user can deposit or retrieve mail. Mail handling system must use a unique addressing system.
Adderssing system used by SMTP consists of two parts: A local part and a global part. The local part is the user name and is unique only within that local mail system. Global part of the address is the domain name. Domain name is identity of the host, must be unique within the total Internet.
.SMTP uses different types of component. They are MIME and POP.
Scenario: Alice sends message to Bob
Alice uses User Agent (UA) to compose message and to bobsinhgad.edu.
Alice’s UA sends message to her mail server, message placed in message queue.
Client side of SMTP opens TCP connection with Bob’s mail server. SMTP client sends Alice’s message over the TCP connection.
Bob’s mail server places the message in Bob’s mailbox. SMTP defines 14 commands. SMTP commands consist of human readable ASCII strings. commands are
More :UPI Remittance Details
HELO : Initiate a mail transaction, identifying the sender to the recipient. ii) MAIL FROM: Tells the remote SMTP that a new mail transaction is
RCPT TO: The sending SMTP sends a RCPT command for each intended receiver.
DATA: If accepted, the sender transfers the actual message. Email
QUIT: Terminate the connection.
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