Verify that the broker has a backlog of undelivered messages by examining its persistent message store in the appropriate directory.
Each file in this directory contains a single undelivered message. If there are more than 10000 files in this directory, then the broker has a backlog of messages. Otherwise, there is another problem with the broker.
The message backlog is probably only log files related to an idsync resync operation and you can safely remove them.
Stop the Message Queue broker as described in Starting and Stopping Services
Remove all files in the persistent message store. The easiest way to remove these files is by recursively removing the message/ directory and then re-creating it.
Restart the Message Queue broker.
Follow the instructions in this section to ensure the broker does not run out of memory again.