Received this error code when sending test emails, client informs that they can send email but not receive.
It appears that the Queue needs at least 4G of free disk space for email to be delivered so: FREE up disk space.
What’s Back Pressure?
In Exchange Server 2007, the Transport service monitors system resources such as disk space and memory on Transport servers (the Hub Transport and the 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 2007 SP1
The Back Pressure settings in Exchange Server 2007 RTM stop inbound mailflow if free disk space is below 4 Gigs. This static threshold has been lowered in SP1 to a more realistic 500 Mb.
The Resolution
Many configuration options for transport servers are saved in an XML file named EdgeTransport.exe.config (same file name on both Edge and Hub Transport servers) located in \Exchange Server\Bin\. You can edit the file to disable Back Pressure, or modify the parameters to more accurately define what’s high utilization for your deployment or server configurations, as explained in the above docs.
Another resolution, and the one I used in this case, was to move the Queue database to another volume with ample of free space, using the following procedure:
Add the following key in the
Restart the MSExchangeTransport service, and message submission resumes
