Hi @yasser Mohamed AbdelMoneim ,
Based on my knowledge, in Exchange 2016 or later, MAPI over HTTP is enabled by default at the organization level, and we cannot configure separately internal and external authentication for MAPI virtual directory. We only could enable Basic or other authentication by Set-MAPIVirtualDirectory with IISAuthenticationMethods parameter. But this will change both internal and external authentication methods.
In addition, according to the Microsoft official article, Microsoft recommend that you always have the virtual directory configured for OAuth.
There is a similar cased you could refer to: How to set MAPI/HTTP internal and external authentication differently
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