Microsoft has provided additional information regarding the issue here.
If you would like our assistance with this problem please let us know, we've been helping clients with this issue all weeekend (standard consulting rates apply unless it's an emergency).
Update 01/02/2022 01:52 AM 18 days
If you're running an on-premise Exchange 2013 or newer, your mail flow may have suddenly and inexplicably stopped. Please follow the link for more information about this Microsoft-related issue (in order to restore your mail flow you need to disable Exchange malware checking which is a potential security risk)
Until you take account of the situation and determine your security impact, remember all ExchangeDefender Pro clients get LiveArchive which will allow you to continue working.