just an idea for discussion:
would a notice/warning for sending a private message just to yourself be appropriate?
we’ve had users send messages to themselves and wondering why they don’t get a response (from the profile page it looks like that’s how you start a new message).
Once they start that process, don’t they notice they can add other people to the recpients? I’m not sure who they expect to reply if they haven’t added people to the message.