Sophos SG230 UTM - Firmware 9.707-5
Was getting Remote Server returned '554 5.0.0 < #5.0.0>' when sending emails from on premise Exchange 2016 server.
To one email domain only. All emails inbound or outbound were sitting in the SMTP spool in the waiting condition.
The errors when retrying says connection refused - totally unrelated to the Country Blocking.
Turns out the email address was a client in Turkey. Turkey was blocked in the Country Block List in Sophos.
Apparently Sophos does not show that error to the user or the IT department.
After unblocking Turkey emails flow normally to this domain.
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