SYMPTOMS

MailEnable server stops responding. Over the network you can ping the system, but that is all, 

no RPD, no mail, etc. At the console it self, the server is extremely slow and non responsive, 

the only way of getting control again is a hard reset (reset switch).

CAUSE

When looking at the event log later, this error is found in the system log every minute:
"The server was unable to allocate from the system nonpaged pool because the pool was empty."

RESOLUTION

This error would point towards a memory leak in an application on your server more particularly you have run out of non paged pool kernel memory.  Please always ensure you are running the latest version of MailEnable then start to look into your other applications  things like Backup Agents, Anti Virus and anything else on the server.

You can monitor the memory in the Task Manager to see if there are any obvious memory increases, you should add all the memory module columns while checking this.

You can use poolmon to see who allocates memory from the pool:
http://support.microsoft.com/kb/177415

MORE INFORMATION

There is also documentation around on the net that discusses increasing the amount of allocated memory in the paged pool it can be overridden manually, as it is bringing your server down you can check this if the diagnosis is taking too long.