The transmit() rule is only called if the triggerTransmission() rule allows transmission. It returns a value of true or MQERETURN_OK. The transmit() rule is called for every remote queue definition that holds messages awaiting transmission. This means that the rule can decide which messages should be transmitted on a queue by queue basis.
A sensible extension to this rule can allow all messages to be transmitted at 'off-peak' time. This allows only messages from high-priority queues to be transmitted during peak periods.