You should be able to do this via admin/customize/email_templates, simply remove the %{message} / %{topic_title} and replace with something like:
posts are private; to view this reply, visit the site.
Looking at the email templates the big ones are:
- User Invited to PM
- User Invited to Topic
- User Linked
- User Mentioned
- User Posted
- User Posted PM
- User Quoted
- User Replied
@neil it might be easier to have a conditional site setting here that suppresses %{message} and %{topic_title}– can you add that to your list?
I do think this matters, for sites that are covering sensitive info, for political reasons or anything else. If post content leaks out through people’s random not-so-secure email services, that defeats a lot of the security of being a private site over HTTPS.