Microsoft Exchange 2007 presents this message in Event Viewer and you are no longer receiving emails. You may also see "Insufficient System Resources" when telnetting to the server.
We found that although the server had 6.75GB of free hard disk drive space it STILL required 'more' to process inbound emails. I ran WinDirStat which pulls out where the space is being eaten up from and deleted 24GB of useless Log files within IIS. A quick restart of the Exchange service (Transport) fixes the problem.
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