'450 4.4.318 SuspiciousRemoteServerError (Full Version)

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andytruelove -> '450 4.4.318 SuspiciousRemoteServerError (4.Jul.2017 6:10:11 PM)


I look after around 40 Exchange Servers at different sites from 2007-2016. In the last 2-3 weeks I've had 5 servers start to not receive eMails from Office 365 users. They bounce back to the Office 365 sender after two days in the retry queue with an error of '450 4.4.318 Connection was closed abruptly (SuspiciousRemoteServerError. The Exchange Servers are literally one of each version (2007,2 x 2010,2013 and a 2016).

I can find several articles on Google all of which say switch off the SMTP deep inspect on a Cisco ASA Firewall. However I don't have one of these in front of any of the Exchange Servers. I do have Watchguard Firewalls in front of them all with an SMTP proxy on which I've tried numerous settings and I'm a litle reluctant to disable this altogether as it's the only a/v & spam block for the eMail. There's no DENY messages in the Watchguard logs either and I've got 30+ other Watchguards with the same config in front of Exchange and these sites don't seem to have the problem.

Just wondered if anyone else had started experiencing this and found any fixes?

Thanks in advance.


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