If the email is in the Gmail's inbox , containing a link to your aspx page => you have sent that email , So you can customize the url like (www.yourpage.com/landingpage.aspx?mail=EncodedEmail) before sending the email
the EncodedEmailwill be the user's email in encoding format ,
on that aspx page you can get teh query string , decode it , get the emailid , i.e. from which email the link is being clicked.
Or you can use open authentication , it needs the user to allow you to access the email id
Check this Solution
Is it possible to get user email using Google api without requesting access to Contacts