Hi @shockoMS ,
Are you running the Exchange Management Shell locally and connecting to your Exchange server using Remote PowerShell as described here?
Connect to Exchange servers using remote PowerShell
https://learn.microsoft.com/en-us/powershell/exchange/connect-to-exchange-servers-using-remote-powershell?view=exchange-ps
Similar issue explained over this thread:
https://community.spiceworks.com/topic/1943690-the-term-get-mailboxpermission-is-not-recognized-as-the-name-of-a-cmdlet
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Best regards,
Leon