Problem: Delivery to clients Exchange Mail server results in a defer message..
dsn=4.3.1, status=deferred (host 18.104.22.168[client public mailserver ip] said: 452 4.3.1 Insufficient system resources (in reply to MAIL FROM command))
Analysis: Most common reason for this error is Back Pressure feature in Exchange Transport Service
Solution: You can reconfigure the Transport service by modifying the "EdgeTransport.exe.config" file which is located in ProgramFiles/Microsoft Exchange Bin folder.
Open Config file with notepad and change resource values or even easier to disable all together.
Example: add key="EnableResourceMonitoring" value="true" /
Change to: add key="EnableResourceMonitoring" value="false" /
Once complete, save the file and restart the Microsoft Exchange Transport service.