Actually I am not a German speaker, but I've just found this. When you try to send a private message to someone, and they have disabled the option, that only their friends can send messages. It says: "%s möchte nur von Freunden private Nachrichten erhalten."
%s, it doesn't mention the name of that person whom I am sending my message to.
I don't know if this must be suggested on German server, since there is no forum for that and I can't speak it. So why not? Thought to post it here! And I guess, it may even happen while sending an inbox or so.