In Microsoft Exchange Server 2019 or Exchange Server 2016, assume that you try to prevent client applications from using REST and EWS by using the EwsAllowList parameter in the Set-OrganizationConfig cmdlet. However, you experience an issue that the application doesn't honor the EwsAllowList parameter. The client application can still use the REST and EWS to connect to Exchange Server even if the value of EwsAllowList is set to null (it prevents all client applications from using REST and EWS).
This issue occurs because all users have the EWSEnabled parameter set to null by default in on-premises Exchange. This setting allows all clients that have null set in EWSEnabled to access without checking EwsAllowList.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
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