Norman- Resolving a '4.3.1 insufficient system resource error' when not receiving email

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, and when resource utilization returns to normal, it resumes message submission. This feature is called Back Pressure. 

When end users are not receiving any inbound mail, and the following message is displayed in the Delivery Queue "return-path address <> rejected by : 452 4.3.1 Insufficient system resources" you will 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 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 space of this volume, you can move the Queue database over to another volume with ample free space by following the steps below:

  1. Add the following key in the App Settings section in EdgeTransport .exe.congig:
    <add key="QueueDatabasePath" value="D:\Queue\QueueDB" />
  2. Restart the MSExchangeTransport service, and the message submission will resume
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 the steps below. Please note that this procedure has not been recommended by Microsoft. 
  1. Using Notepad, open the EdgeTransport.exe.config file from \Exchange Server\bin directory
  2. Add the following key+value pair:
    <add key="EnableResourceMonitoring" value="false" />
  3. Save the file
  4. Restart the Microsoft Exchange Transport Service (MSExchangeTransport)

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