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.