Resolution:
This can happen when an application which is sending messages to an EMS server does not commit transactions. Before a transaction is committed, messages within a transaction are stored in EMS memory. They are not shown as pending messages and can not be removed by the "purge" command. They will not be sent to a consumer as normal pending messages.

"show connections full" may show information such as the  following:

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J  6.3.0 V5  21089 +--T  +  Host 10.x.x.x testuser                     2    2    0    0    0 3756821     5.1 GB    4:15:30

Check the output of the "Uncomm" and "UncommSize" columns.

Uncomm       - (optional) messages in uncommitted transactions on connection.
UncommSize - (optional) bytes in uncommitted transactions on connection.
 
The above output shows that the number of uncommitted messages is 3756821 and the size of those messages is 5.1 GB.
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Run "show stat producers" to check information of producers.

Enabling client_trace in the EMS server and review the EMS client traces from the application side covering the problem period.

Refer to the EMS User's Guide for more information on client_trace.

For other scenarios related to high MMU, refer to KB 25816.