My end users aren’t receiving any inbound mail and when I check Delivery Queue, the following message is displayed in my mail:
"return-path address <email@example.com> rejected by mail.mydomain.com:25 : 452 4.3.1 Insufficient system resources"
In Exchange Server 2007, the transport service monitors system resources such as disk space and memory on the transport servers (the Hub transport and Edge transport servers), and stops message submission if it is running low on these resources. It continues to deliver existing messages in the queue. When resource utilization returns to normal, it resumes message submission. The feature is called Back Pressure.
To correct this issue, you need over 4 GB of free space on the volume where the Queue database is located.
In Exchange Server 2007 SP1, this static threshold has been lowered to a more realistic 500 MB.
Note: Configuration options for transport servers are saved in the XML file, EdgeTransport.exe.config, which is located in the following directory (\Exchange Server\Bin\).
To free up some space on this volume, you may move the Queue database over to another volume with ample free space by following this procedure.
Note: The Back Pressure feature, which detects resource pressure on Exchange Server 2007 transport servers and stops accepting new message submission if low on resources, may be turned OFF by following these steps. Please note that this procedure has not been recommended by Microsoft.