Basically i want to make an app which will check that user has opened the sent mail or not
just like the script of "[http://www.readnotify.com][1]" like we have to just add
readnotify.com after the receivers email address before sending the email.
i have read that it is done by adding the img tag with very small size(1*1) in the email
<img src="http:/yoursite.com"/>
something like the above code and we got even the geolocation of the receiver.
How are they doing this behind the scenes??
means what they are doing in their PHP code ??
Image is generated by your script, which reads $_SERVER variables ( http://php.net/manual/en/reserved.variables.server.php ). Then you can get clients IP and so on. But rest assured, that most of mail clients do not open images unless user permits it beforehand.