This isn't a feature that email has, though no doubt some one will have put it in as a feature, as scott_chegg says it relay is down to the email server to do that. You can do other things like imbed remote images and track there access, but there isn't a way to grantee that the information it returns will let you know if the email is been accessed by some one else. Copy and pasting the content of an email will allow this method of tracking to persist if not done correctly.