Abstract: A system and method for filtering undesirable e-mail with forged nonexistent sender addresses in real time without sending a message to that sender. A server or proxy rejects electronic messages having a nonexistent sender address by establishing a connection to an authorized mailhost for the domain of the sender address and using discrete transactions during the protocol setup phase to determine if that mailhost will accept mail for the sender address. If the authorized mailhost for the sender address will not accept mail for that address, then the sender address is determined to not exist and the message can be safely rejected.

Abstract: An Active Filtering proxy filters electronic junk mail (also known as spam, bulk mail, or advertising) received at a Message Transfer Agent from remote Internet hosts using the Simple Mail Transfer Protocol (SMTP). The proxy actively probes remote hosts that attempt to send mail to the protected mail server in order to identify dialup PCs, open relays, and forged email. The system provides multiple layers of defense including: connect-time filtering based on IP address, identification of dialup PCs attempting to send mail, testing for permissive (open) relays, testing for validity of the sender's address, and message header filtering. A sender's message must successfully pass through all relevant layers, or it is rejected and logged. Subsequent filters feed IP addresses back to the IP filtering mechanism, so subsequent mail from the same host can be easily blocked.