This invention relates to a method and apparatus for blocking spam messages originated in a different network. An anti-spam check is made in response to an initial Send Routing Information (SRI) message received in a destination network of a message. The result of this check is sent back to the originating network along with a destination address of a switch in the destination network, and SRI spam code and Mobile Terminated (MT) Routing Address Correlation ID. If SRI spam code indicates a spam, then subsequent transmission of the MT message with the same Correlation ID is blocked in the interworking switch or SMSC.