When starting the EMS server you will get something like the following sequence:

2007-07-03 03:48:16 Recovering state, please wait.
2007-07-03 03:53:38 Recovered 805962 messages.
2007-07-03 03:53:39 Server is active.
2007-07-03 04:30:43 Memory limit of 512 MB exceeded.
2007-07-03 04:30:43 Memory limit of 512 MB exceeded.

Why do I get "Memory limit of 512 MB exceeded", what should I do for this problem?

Resolution:
This means that the EMS server ran out of max_msg_memory limit.

You should investigate why so many messages were accumulated in the EMS server.

Either your upstream sending clients are flooding the EMS server and the messages are not being taken off the EMS server with your downstream receiving clients quickly enough.  
Or some of your downstream receiving-clients are too slow or offline for long periods of time which would mean that your EMS server is being used as message-storage.  

Please run the following tibemsadmin commands to investigate which destinations are of problem. Are the consumers running. Which applications are potential the problem applicaitons.

show server
show connections full
show consumers full
show queues
show topics
show durables