Most of my clients check their email with Outlook which still, unlike Gmail, loads images. Is it a possible future feature to have a little invisible image in the followups that get sent and use that to tell if the emails have been opened or not? Using this, I can tell if I should just cancel a client who's ignoring my followups or if there's a real problem that needs to be addressed (maybe their email is entered wrong - so I could phone to followup)