Abstract: This invention concerns electronic message authentication, such as email messages, to ensure valuable messages are reliably delivered to the recipient, while reducing the delivery of unwanted messages. The invention involves: Receiving an electronic message addressed to a recipient. Rejecting messages sent to unknown recipients, from compromised machines or otherwise found invalid. Testing the messages to valid recipients to determine whether the status of the sender of the message can be categorised as trusted or not-trusted. If the status of the sender cannot be categorised either way, then automatically sending a challenge message, and holding the received message pending receipt of a reply. If an acceptable reply is received, categorising the sender as trusted. And, if the sender is categorised as trusted, delivering the message to the recipient.