The SMTP Protocol

Email is one of the most popular uses of the Internet and is the main reason for manyto have adopted it.
Email allows you to send a message to anyone on your own network or anywhere else in the world using the Internet.
Email software also provides options whereby a reader could read print reply to and store email.
The Format of a Email Message
An email message consists of two parts:
The first part is known as the header which includes information:
From whom the mail has been sent
From: somebody@india.com
The person/people to whom it has been sent
To: anybody@america.com
A copy of the mail can be send to as many other people as you like at the same time as you send it to the main recipient.
Just enter the full email addresses in the cc box separated by commas or semicolons.
Cc: everybody@europe.com everybody@australia.com
If copies of the email need to be sent to multiple recipients but their email addresses must not be displayed then use Blind Carbon Copy (Bcc)
Bcc: hidden@someplace.com
The subject of the message
Subject: Project confirmation
Note: Some header information is mandatory (like the ones mentioned) whereas others are optional.
The second part is known as the body and contains plain ASCII text.
The body of the message follows the header and is usually separated by a blank line.
Email messages can only transmit plain text.
There are however encoding processes that extend the functionality of email by allowing the transfer of other data like images in the form of an attachment.
After typing your message click on Send.
Electronic Mail Addresses
An email address is always in the form of 'mailbox@Domain-name' where mailbox refers to a user or other recipients (such as distribution lists) and the domain name represents the domain of which the email server is a member.
Examples of an email address could be sudhir@jetking.com where 'sudhir' is the mailbox name and 'jetking.com' is the domain name.

SMTP Protocol
The SMTP protocol specifies how mail should be delivered from one system to another.
This protocol makes the connection from the sender's server to that of the recipient and then transfers the message.
SMTP is used:
To deliver messages from the email client to the SMTP server.
To transfer messages from one SMTP server to another.
The sending SMTP server discovers the IP address of the recipient SMTP server using the domain name part of the email address.
The SMTP server is registered on the DNS using a 'mail exchanger' (MX) record.
SMTP is not used for transferring messages from the recipient's SMTP server to its email client because it requires both source and destination to be online to make a connection.
An SMTP server retries regularly before it returns a Non-Delivery Report (NDR) to the sender.
Working of SMTP
This is based on the model of communication which is a result of a user mail request.
Because of this the sender-SMTP establishes a two-way transmission channel to a receiver-SMTP.
The receiver-SMTP may be either the ultimate destination or an intermediate.
The SMTP commands are generated by the sender-SMTP and sent to the receiver-SMTP.
SMTP replies are sent from the receiver-SMTP to the sender-SMTP in response to the commands.
After the transmission channel is established the SMTP-sender sends a MAIL command indicating the sender of the mail.
When the SMTP-receiver accepts the mail for that recipient it responds with an OK reply.
In case it does not accept the mail it responds with a reply rejecting that recipient.
Post Office Protocol (POP)
SMTP delivers mail only to hosts that are currently available.
POP is therefore designed to allow the recipients to download their mail to their email client at their own convenience.
One of the most widely used methods for retrieving electronic mail from a mailbox on computing devices is called the Post Office Protocol (POP).
POP has been adopted as a standard for email exchange by virtually all Internet service providers worldwide.
A POP client like Eudora or Outlook connects via TCP to your POP server.
The POP service is different from the SMTP service but runs on the same server.
The user is authenticated by way of a username and password and your mail is downloaded accordingly onto his PC.
Sending Email
The process of sending email can be described as follows:
The email client software on the sending computer sends the message to the email server
The email server notes the recipient's address e.g. carl@prestige.com.
It then contacts a DNS server to find the IP address of the domain name which is mentioned in the address i.e. prestige.com
It then uses SMTP to deliver the message to this email server.
The delivery requires several hops.
The World Wide Web: HTTP
The World Wide Web (WWW) is made up of a huge number of servers that can be accessed by entering the correct URL address.
Hypertext and HTTP (HyperText Transfer Protocol) form the basis of this Web.
This information can be accessed in any order and from anywhere using the hyperlinks provided.
These links are contained in the document and contain the URL of another location.
A link can also refer to another document or a location within a document.

Links are not necessarily confined to information on the same site.
For example:
http://www.jetkinginfotrain.com/
This site address takes you to Jetking Institute home page.
Links to other documents are present on this page.
The link Jetking "Kal Aaj aur Kal" requests the document found at http://www.jetkinginfotrain.com/kal_aaj_kal.htm.

They can also link to sites on other servers companies and even continents.

For example
In a search engine such as Google the documents that match your search criteria are displayed as links and may be on any server anywhere in the world.
The Structure of a URL
The Uniform Resource Locator or URL as it is commonly known is used as the addressing scheme of the Internet and other browser-based application.
The URL contains all the information that is required to locate any resource.
Each URL is unique and no two different Web pages or location can share the same URL.
The URL has a fixed structure and can comprise of up to six components.
Capitalisation is important in URLs especially since many Web servers regard upper and lower case alphabets as different.
This particularly holds true in the case of UNIX servers.