The Sharing policy doesn’t allow you to grant certain permissions to users on CalendarFolder folder “Calendar”

When attempting to share Calendars between users via powershell, I encountered an error stating:

The Sharing policy doesn’t allow you to grant certain permissions to users on CalendarFolder folder “Calendar”

Sharing Policy Doesnt Allow You To Grant Certain Permissions

The first cmdlet I used was:

Add-MailboxFolderPermission -Identity nick.lastname@domain.com:\Calendar -User claire.lastname@domain.com -AccessRights Author

This returned the error above.

I tried sharing again, referring to the users with just their aliases:

Add-MailboxFolderPermission -Identity nick.lastname -User claire.lastname -AccessRights Author

This resulted in the following error which points to the real issue:

The specified mailbox “nick.lastname” isn’t unique.

Specified Mailbox Isn't Unique

In this tenant, we have multiple users with the same alias and different domain names – eg. nick.lastname@domain1.com and nick.lastname@domain2.com

The Add-MailboxFolderPermission cmdlet might require that the users have unique aliases. To resolve this, I changed the user’s alias to something unique using the Set-Mailbox cmdlet

Set-Mailbox -identity nick.lastname@domain1.com -Alias nick.lastnamedomain1

Change Alias To Add Permissions

Once this was resolved for the affected users, I could add the mailbox permission without issue by referring to the users by their unique aliases:

Add-MailboxFolderPermission -Identity nick.lastnamedomain1:\Calendar -User claire.lastnamedomain1 -AccessRights Author

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