You can't tell if a user has read an email, unless you provide a link for him to click and he clicks it - this is because spammers and phishers would like to know "live" email addresses so most systems are set to not download content (images etc) or send read receipts. Most systems (such as this site) send the user an email when he signs up, which contains a link to confirm the email address. Until the link is clicked, the user is restricted in what he can do (if anything). The easiest way to do this is to provide a "confirmation" page and supply a link with a querystring which identifies the user via a GUID value or similar to prevent he manually "guessing" the right string to use. The GUID provides a lookup which translates to the user so the registration DB can be updated.
To read gmail emails, see here:
Read Gmail Inbox Message in ASP.NET[
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