When using mail most people don't want to let know when they read a mail. I do for instance because it hurts my privacy. If the notification was delayed so that the sender doesn't have the precise time when I red the mail, it could reduce the problem. Though in other context, like intra-work, beeing able to know when someone opened a mail might be of value for both ends.

Add to the equation the spammers who would be delighted to know if their mail was red.