Each queue has a suffix of Rpriority_number, where R1 is the higher priority. The message queues are treated as FIFO (First In, First Out), and new messages are always added to the end of the queue.
The Exchange Mailbox task will process the queues in order of priority. The task scans through each queue, starting with the highest priority. If it finds a message on a queue, it will process the message and then start the scan again from the highest priority queue. Therefore, if there are messages on queue R1, queue R2 will not be processed until queue R1 is empty.
If a queue has not been processed for more than 10 minutes, and there are no messages in a higher priority queue, there may be a problem with the task. Check the Enterprise Vault event log on the Exchange Mailbox task computer for any additional information.