Retrieve Email Id from outlook and send as a response when the user opens the mail

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ASP.NET

Hi,
Am sending mails to a group in outlook,the group consists of many users and when the user opens the mail i want to know which user is opened that mail and send it as a response. Currently am adding an image in the mail so on load of the image am calling some url where i can know that some user has opened the mail but i need email id as a parameter to the url. Could you please let me know how can we do this in outlook to get the user mail id. do we need to write any macro or anything??

Thanks
Ranjith