Mark All Messages as Read is a dangerous drop-down menu item if you intentionally keep unresolved e-mails unread until they are resolved. A quick ESC or undo can fix it, but I recommend setting this up so that an “Are You Sure? Yes/No” option occurs before the client begins marking all messages as unread.
I just had that issue, I didn’t know ESC could stop it (albeit not reverse it) and undo could not fix it since undo was grayed out… I had to restore a backup which is now costing me around 10 minutes of my day already.
While I cannot speak to the technical feasibility of a reverse/undo action, a simple (maybe even double - as you said, “unread” is often an important attribute) confirmation dialogue is certainly a low hanging fruit.