System and method for delivery of an electronic message from a sender unknown to a recipient when such message comprises a sender code verified by recipient's electronic message handler, thereby eliminating delivery of unsolicited bulk messages. Dispensing sender codes per recipient addresses thwarts senders of unsolicited bulk messages. Dispensing sender codes upon successful response to a human response question further thwarts bulk senders. Sender code calculation according to periodically changing factors limits sender code utility to finite time periods.