For a recent support request, I wanted to explain the different security level settings for who can send you private messages, and looked for an appropriate FAQ, but didn't find one.
How about something along these lines:
Your Inbox is also where you send and receive private messages from other LiveJournal users. Writing a new message to another LiveJournal user is as simple as clicking the "New Message" button and filling out the form.
You can restrict who can send you messages at the Edit Profile page (Contact Info > LJ User Messaging).
- It is also possible to ban individual users from sending you private messages by banning them from commenting in your LiveJournal.
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