Is there a way to only mark the most recent email in a thread as unread? Right-click -&gt; Mark as Unread marks every email in the thread as unread.
I am using MacOS 10.14.6 and Mail 12.4.
In a message thread in the mail app, there's a small indicator of how many messages are in the thread:

If you click on that indicator, it'll open up the thread and show all the individual messages:

You can then interact with the individual messages, including setting the most recent one as unread.