Double check all email
Here's my recipe for reducing spam.
a) in addition to the purported sender, as at present, every sent message is required to have an extra header containing a unique sender reference number and a content checksum.
b) as the message passes through each server on its way to the recipient, it recalculates the checksum and sends a small datagram back to the purported sender, asking it to confirm that the sender's server really did send the message.
c) If it doesn't get positive confirmation, it doesn't pass it on but instead sends it to whoever is registered as providing internet connectivity to the server which forwarded it.
Performing the check by EACH server along the route ensures that the perpetrator can be more reliably identified -- if server C receives a spam which server B purports to have come from server A, it knows somebody in server B's jurisdiction is in collusion with the spammer and probably forged the headers, because B should have rejected the message if it couldn't get positive confirmation from A.