Most of us believe that the email message sent to them is determined by "FROM" header,but it cannot be denied that most of the times it's forged(Learn How).
The question then arises how to determine where the email came from? You have to understand how email messages are put together in order to backtrack an email message. SMTP is a text based protocol for transferring messages across the internet. A series of headers are placed in front of the data portion of the message. By examining the headers you can usually backtrack a message to the source network, sometimes the source host.
If you are using Outlook or Outlook Express you can view the headers by right clicking on the message and selecting properties or options.If you are using Gmail you need to click on "show original"(click on down arrow next to reply while reading a mail).For Yahoo:In the full message view,"View Full Header" option is placed under the "Actions" tab.
Now Let's Start actual work.Let's Take an example