Restricting mail


I was hoping someone could give me an answer to the following question. I was looking through all the settings in elgg and could not find something to costomize how mail is send. How can i make that only friends can send mail to each other. At the moment anyone can send mail to anyone, even when not friends. Is there a way to customize this area? Please anyone that know of a way and that could help as this is a concern for my site setup.

Thank you in advance