In a system and method and system for controlling delivery of unsolicited electronic mail messages, one or more spam probe e-mail addresses are created and planted at various sites on the communications network in order to insure their inclusion on large-scale electronic junk mail ("spam") mailing lists. The mailboxes corresponding to the spam probe e-mail addresses are monitored for incoming mail by a spam control center. Upon receipt of incoming mail addressed to the spam probe addresses, the spam control center automatically analyzes the received spam e-mail to identify the source of the message, extracts the spam source data from the message, and generates an alert signal containing the spam source data. This alert signal is broadcast to all network servers and/or all user terminals within the communications network. A filtering system implemented at the servers and/or user terminals receives the alert signal, updates stored filtering data using the spam source data retrieved from the alert signal, and controls delivery of subsequently-received e-mail messages received from the identified spam source. The filtering system controls delivery of the unsolicited e-mail messages by discarding the messages without displaying them to the user, displaying the messages to the user with a "JUNK" or similar marker, or otherwise processing the spam mail as desired by the network provider and/or the network users. The filtering system may also filter e-mail messages sent by the user terminals.