I have had the same issue with replies for private categories with hundreds of people.
A user sent his resume to the group saying he was looking to leave his current job when he thought he was just replying to the last poster. I have had committes refuse to use private categories or PMs because they had no idea who was getting the responses.
The default original email looks like it comes from the user with an image even. You have to check the email address to know that the message isnt a personal email. The email is not visible on mobile The subject line is some help.
Consider the following
View Topic* or reply to this email. Your response will be published to the group.
I also add a footer with a link to reply privately and other items. A key reason was to highlight: Yo! This is not an email.
The same issue is present for group PMs. A better footer would help.
I like that Discourse starts with basic text that can be customized but I think there are some pitfalls out there for admins and users.