I can understand SE's desire to block unlimited mail between random strangers. (I'm glad it keeps RMT spam out of it.) But they went a bit too far in making the rule be Friends only. I think a more appropriate rule would be that you can send mail to Friends, FC members, and alts.

While some might benefit from a larger Friends list, just being able to mail FC members without duplicating them in that list would probably cut down the demands on it far enough for most players.