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Exchange 2016 intermittently refusing messages but accepting them after retries

Environment: Windows 2016 Standard running Exchange 2016 (15.1 ‎Build 2308.8). All critical and important updates installed. HealthChecker.ps1 shows it is healthy.
Issue:
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!

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@JaxPlanet-9550
I am writing here to confirm with you any update about this thread now?
If the above suggestion helps, please be free to mark it as an answer for helping more people.

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KyleXu-MSFT answered KyleXu-MSFT edited

@JaxPlanet-9550

I would suggest you try to bypass your mail filtering service and check whether this phenomenon gone.

If this phenomenon, it means this phenomenon may related your mail filtering service setting.

If this phenomenon still exist, I guess it may caused the the back pressure(Mainly caused by high email send/receive rate). You could have a check about whether there exist event log about this phenomenon from event viewer.


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JaxPlanet-9550 avatar image
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JaxPlanet-9550 answered

I just found out the vendor upgraded the firewall, which wiped mail filter's CIDRs from the bypass list. The interference from the firewall is what caused the intermittent deferrals. I put them back on the list and all is good.

I am going to mark Kyle's response as the answer because it's still a good place to start troubleshooting this problem.

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