How do I send an email address to someone @ facebook.com without having to put it in the Other Messages folder?

Facebook filters many messages in the Other Messages folder (http://betabeat.com/2011/12/return-of-laptop-lost-in-taxi-foiled-by-facebooks-obscure-message-folder/).

Does anyone know how they filter incoming mail to @ facebook.com email addresses? Can non-friends email users on @facebook? Can applications?