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"
To correct this issue, you need over 4 GB of free space on the volume where the Queue database is located.
Within your MSExchangeTransport, you will find an Application Event Log. The Event Properties Description dialog box will display the following message “The resource pressure is constant at High. Statistics... ". The description will also state that inbound mail submission has stopped, and it's due to disk space being low on the volume where the queue database is located.
Image: Event ID logged by MSExchangeTransport
What is Back Pressure?
In Exchange Server 2007 to 2016, 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.
Changes to Back Pressure Settings in Exchange Server
The Back Pressure settings stop inbound mailflow if free disk space is below 4 GB. This static threshold lowered im 2014 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 some space on this volume, you may move the Queue database over to another volume with ample free space by following this procedure.
Add the following key in the <AppSettings> section in EdgeTransport.exe.config:
<add key="QueueDatabasePath" value="D:\Queue\QueueDB" />
Restart the MSExchangeTransport service, and then 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 these steps. Please note that this procedure has not been recommended by Microsoft.
Using Notepad, open the EdgeTransport.exe.config file from \Exchange Server\bin directory.
Add the following key+value pair:
<add key="EnableResourceMonitoring" value="false" />
Save the file.
Restart the Microsoft Exchange Transport Service (MSExchangeTransport).
Make a backup copy of the edgetransport.exe.config file first.
Using Notepad, open the EdgeTransport.exe.config file from \Exchange Server\bin directory
Add the following key+value pair:
<add key=”EnableResourceMonitoring” value=”false” />
Save the file
Restart the Microsoft Exchange Transport Service (MSExchangeTransport):
Move the queue database to another volume which has lots of space. Add the following key to change the path of the QueueDB.
Using Notepad, open the following file: C:\Program Files\Microsoft\Exchange Server\V14\Bin\EdgeTransport.exe.config.
In the <appSettings> section, modify the following line:
<add key="QueueDatabasePath" value="<LocalPath>" />
This example creates a queue database at the location C:\Queue\QueueDB.
<add key="QueueDatabasePath" value="C:\Queue\QueueDB" />
Save and close the EdgeTransport.exe.config file.
Restart the Microsoft Exchange Transport service.
Verify that the new Mail.que and Trn.chk files are created at the new location.
Remove the unused Mail.que and Trn.chk files from the original location.
Please refer to the following for more information: