The Sender initiates an Email Acknowledgement which activates when the Recipient opens the email.
You wouldn't want it to be activated for EVERY email that the User opens - otherwise it would also confirm a valid email address if the email was sent by a Spambot or was being used to check which emails were active and which were not. Scammers and Phishing attacks are done this way, hoping that the recipient will be stupid enough to reply to what is basically an enticing (but totally unexpected) email.
Would this User really want to send acknowledgements to someone that was trying to find out their personal (including email address) details? I seriously doubt it. ;)