Today we were contacted about a Small Business Server problem where emails were not arriving.

The usual checks showed that Internet Connectivity was OK, and there was nothing obvious showing in the event logs.

MXToolbox returned no problems on the SMTP check… so I¬†decided to run the Exchange Troubleshooting tool for inbound SMTP to see if there were any errors thrown.

Sure enough, an error was returned Р Exchange Server Transport error: 452 4.3.1 Insufficient System Resources

A quick check showed plenty of free disk space, but the Ram was running at over 90% utilisation.

This post indicated a feature of Exchange that dealt with Back Pressure – where inbound email is blocked when Exchange monitors show server resources are low.

In this instance, I modified the EdgeTransport.exe.config located in the Exchange Server\Bin folder to set the Resource Checking to “false” and restarted the Exchange Transport service, which had the effect of disabling back pressure.

This had the immediate effect of restoring email service and allowed me time to set the thresholds to more appropriate values.