I responded to this question partially on this post which was also on enewsletters: https://www.innspiring.com/node/2123#comment-24741
Happy to follow up if necessary.
Email opens usually have a steep drop off after the first 24 hours unless there is something on it that compels users to keep it like a coupon or a special event.
Re: your first post- Email opens usually tracked using tiny transparent image in the body of the email. When a user opens the email...