Excessive message queue growth may indicate that messages are not being delivered, are being delayed in their delivery, or are coming in faster than the system can deliver them. This may be caused by a number of reasons such as a denial of service attack caused by huge numbers of messages flooding your system, or the Job Controller not running.
Disk space usage grows.
User not receiving messages in a reasonable time.
Message queue sizes are abnormally high.
Probably the best way to monitor the message queues is to use imsimta qm and imsimta summarize. Refer to 27.8.6 imsimta qm counters.
You can also monitor the number of files in the queue directories (msg-svr-base/data/queue/). The number of files will be site-specific, and you’ll need to build a baseline history to find out what is “too many.” This can be done by recording the size of the queue files over a two week period to get an approximate average.