I encountered this error on my on-premise/Hybrid exchange server today after noticing that internal mail relay was not working. When attempting to send a test email via telnet, I got the error 451 4.7.0 Temporary server error. Please try again later. PRX5
After rebooting the server, the error still persisted - so a quick bit of searching quickly lead to me adjusting the Exchange servers DNS settings.
From within Exchange Control Panel (ECP) - go to Servers (left hand menu) and double click your Exchange server to open it's properties
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Select DNS Lookups from the left menu.
By default, the option for External and Internal DNS lookups is usually All network adapters (All available IPv4). I changed this to Microsoft Hyper-V Network Adapter and the problem was immediately resolved.
