How to stop Mail from automatic "mark as read" behavior? (in Sierra)
Solution 1:
"Mail Act-On" by SmallCubed has a feature to "Do not mark message as read". It currently costs $30 and that is a lot if that is the only feature you plan to use (it's main purpose is to help you create advanced mail processing rules). I have used it in the past and it worked well (currently using Outlook).