Another issue affecting onPrem Exchange servers (bug affect 2016 & 2019) has been made public today where emails are trapped in the transport queues, this is related to a date check failure with the change of the new year. Microsoft has confirmed this is not a security related issue.

Microsoft is working on a patch and has released a workaround posted here.

[1] https://techcommunity.microsoft.com/t5/exchange-team-blog/email-stuck-in-transport-queues/ba-p/3049447
[2] https://social.technet.microsoft.com/Forums/exchange/en-US/136b0705-6326-42c0-bff0-a6412fc84fb2/message-deferred-by-categorizer?forum=exchangesvrsecuremessaging
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Guy Bruneau IPSS Inc.
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gbruneau at isc dot sans dot edu