Environment: Windows 2016 Standard running Exchange 2016 (15.1 Build 2308.8). All critical and important updates installed. HealthChecker.ps1 shows it is healthy.
Email is sent to Internet mail filtering service and relayed to on premise Exchange server.
On premise Firewall forwards emails from mail filtering service IP ranges to Exchange server.
Intermittently, Exchange refuses messages from filtering service. I know this because it is stated in the filtering service logs but no reason is provided. In almost every case, the refused messages are delivered after retries so it is not a "permanent" failure.
I have been trying to find Exchange logs that might provide more insight into why these temporary refusals are happening but haven't had much luck.
Anyone have any ideas why this is happening or where I can look for answers?
Thank you in advance!