A method is provided for filtering electronic messages such as unsolicited bulk email. The method includes establishing a communications connection with the remote server under the Simple Email Transfer Protocol or other suitable protocol; accepting session data from the remote server; and determining if the session data meets criteria associated with legitimate messages. If the data pertaining to the connection does not meet the criteria associated with legitimate messages, then one or more stimulus signals are sent to the remote server. If the server responds to the stimulus signal in the manner required by the communications protocol, then the session can be added to a database of known sessions. The session is then terminated by responding to the server with a temporary error message in accordance with the protocol. If the remote server is a legitimate server complying with the STMP, it will resend the message after a prescribed time period.