You probably need a mailer service such as mailchimp or constant contact, who lace their messages with scripts and webbugs to report home. Savvy users can still circumvent these measures by opening messages when offline or in plaintext mode. Ultimately, whatever you do depends on the co-operation, whether consciously or unwittingly, on the part of the recipient and their chosen email client. Both concerns place it outside the control of what you send it with.