The spec doesn't provide any facility to allow an entity to determine if the entity the message is intended to be forwarded recognizes forwarded messages. Such a facility would be useful, I would think, as a client which doesn't support this extension will ignore the <forwarded/> content. The forwarding client can use discovery to determine if support is lacking and, if so, take appropriate action, such as indicate to the user that the forwarded content might not be presented by the client they are forwarding it to.
-- Kurt
