According to the user instructions, unregistered users can send PMs to members using uddeim when use as part of CB, but unfortunately I cannot get it to work!
I have set "enable public frontend" to "yes" in the uddeim admin "public" tab, and I have set the relevant private message tab in CB for "public" access, but I still cannot see the private message tab in the frontend unless I login to my site first. Now I'm stuck - no idea what to do.