The spam blocker monitors the SMTP/TCP/IP conversation between a sending message transfer agent MTA.sub.--0 and a receiving message transfer agent MTA.sub.--1; catches MTA.sub.--0's IP address IP.sub.--0, MTA.sub.--0's declared domain D.sub.--0, sender_address A.sub.--0; and recipient A.sub.--1; and uses this source and content based information to test for unsolicited messages. It interrupts the conversation when MTA.sub.--0 sends a command specifying the recipient (an "RCPT" command) and uses the various test results to decide if the message is suspected of being unsolicited. If the message is suspected of being unsolicited then it logs the rejected message, sends an error reply to MTA.sub.--0 which forces MTA.sub.--0 to terminate the connection with MTA.sub.--1 before the body of the message is transmitted; else it logs the allowed message, releases the intercepted RCPT command which allows the conversation between MTA.sub.--0 and MTA.sub.--1 to proceed.