Can you describe in more detail how you have set this up? What do you mean by private topics?
What do you see when you click on the envelope in the top right corner of posts as in the screenshot? You should be able to see the source email there, including details that will help you with troubleshooting what is going on with their emails. I suspect there’s not going to be alot you can do about it if they are not sending emails with properly formed headers.