In my experience, users often do not understand that a reply from the original author to a private reply is public.
Would you be open to trying a slightly different algorithm optionally?
- Replies to private replies are also private
Replies to the first topic are not private- Only replies of the original author to his first post or other public post are not private
Implementation-wise, adding an extra field to posts to mark them as private replies would also enable another experiment I would like to try:
Quite a few of our users, who digest the forum only by email, are not aware that a reply to an email goes public in the forum (although we added a footer with that information). I would like to try switching any reply by email to private in selected categories.