I don't know if something like that already exists but at least google couldn't find anything like that for me.
I would love to have a function / AddOn which a member can press within a thread and send it out via his local mailclient. A sort of mailto:emailadress, subject: This could interest you, mailtext: Linkt to thread
I guess this would be a nice to have for any admin as it pushes also non member to register and increase number of members....
Maybe there is already such a feature in the core and i was just blind....in this case....please tell me where to find and delete this...