How can email addresses of the form
firstname.lastname@example.org be used?
Is it possible to send normal mails to those addresses that are forwarded somewhere or are there deeper restrictions?
These used (?) to be handled for Facebook applications as a proxy so users do not have to release their email address publicly. Facebook assigns one when granting access to an application and the user requests an anonymous email instead. A temporary one is randomly generated for the user to receive emails from the application developer.