Block MapiOverHttps from external only
Solution 1:
Based on my knowledge, you could not configure MapiOverHttps internaly, and OutlookAnywhere for external connections. Detailed information in the below screenshot and the blog for your reference.
And we cannot configure external or internal authentication for MAPI virtual directory, we only can enable Basic or other authentication by Set-MAPIVirtualDirectory with IISAuthenticationMethods parameter. Refer to the similar thread: How to set MAPI/HTTP internal and external authentication differently