To enable multiple message originators to share an SMS shortcode, a service provider that administers the shared shortcode determines which of the recipients of an outbound SMS message have subscribed to SMS services of the particular originator. Only messages to recipients that have specifically opted in to receive messages from the particular originator are sent by the service provider through an SMS gateway. To opt-in to SMS message services, a user sends an opt-in message to the service provider. The content of the opt-in message includes a code that uniquely identifies one of the plurality of originators.