Symptom: Your email gateway server is unable to deliver messages to your backend Exchange mail server because of an Insufficient system resources error.

Source: This is an Exchange-specific issue, which happens when less than 4 gigs of free disk space remain on the Exchange servers queue drive for a particular domain.

Here’s a sample log snippet from our own gateway server:

Analysis: This is likely caused by the back pressure feature in Exchanges Transport Service. In Exchange Server 2007, the Transport Service monitors system resources such as disk space and memory on the Transport servers (e.g., Hub Transport and Edge Transport servers), and will stop new message submission if these resources run low. Existing messages in the queue will continue to be delivered, and when resource utilization returns to normal, message submission resumes. This feature is called back pressure.

Solution: To remedy the issue, you will need to reconfigure the Transport Service by doing the following:

1- Open the EdgeTransport.exe.config file, which is located under the C:Program FilesMicrosoftExchange ServerBin directory.

2- Disable resource monitoring or change the monitoring configuration:

Open the EdgeTransport.exe.config file with Notepad and change the resource values; or an easier option is to just disable the monitoring completely.

To change the resource values, the string should go from: add key=EnableResourceMonitoringvalue=true/
To add key= EnableResourceMonitoringvalue=false/

Once this is done, save the file and restart the Microsoft Exchange Transport Service.