the origin address of an e - mail message is validated to enable blocking of e - mail from spam e - mail sources , by preparing , in response to the receipt of a predetermined e - mail message from an unverified source address , a data key encoding information reflective of the predetermined e - mail message . this message , including the data key , is then issued to the unverified source address . the computer system then operates to detect whether a response e - mail message , responsive to the challenge e - mail message , is received and whether the response e - mail message includes a response key encoding predetermined information reflective of a predetermined aspect of the challenge e - mail message . the unverified source address may be recorded in a verified source address list . thus , when an e - mail message is received , the computer may operate to accept receipt of a predetermined e - mail message on condition that the source address of the predetermined e - mail message is recorded in the verified source address list and alternatively on condition that the predetermined e - mail message includes the response key .