Control of message delivery from publisher devices to a subscriber application of a messaging system is provided. The subscriber application has a subscription registered with a broker application of the messaging system and is only permitted to receive messages relating to a topic of the subscription from a predetermined set of one or more publisher devices. The method includes: altering a topic identifier of a message from a publisher device to a modified topic identifier, and altering a subscription identifier from a subscriber application to a modified subscription identifier; analyzing the modified topic identifier and the modified subscription identifier to determine if the message is to be communicated to the subscriber application, and if so, information is removed from the modified topic identifier to form a reverted topic identifier, and the message and reverted topic identifier are communicated to the subscriber application.