I am working on a spam filter. I want both incoming and outgoing messages to go through the filter, not because the outgoing messages need to be filtered, but because I want the filter to know who my authorized users have sent messages to because that is a very reliable indicator of non-spam. My setup requires users to authenticate, so postfix knows who they are. My question is: is there a reliable way to pass this information to a filter? I can't find anything about this in the documentation. Reverse engineering indicates that postfix puts an "Authenticated sender" note in the received-from header, but that can be forged. Is there a reliable way for a filter to tell if a message is from an authenticated user?
Thanks, rg
