Can't add permissions to Outlook user or room mailbox in another forest in Microsoft 365 hybrid environment

Original KB number:   3195201

Symptoms

In Microsoft 365, you can't add a security group or user to the permissions of a cross-forest room (or shared) mailbox in Microsoft Outlook. Specifically, this issue occurs in the following scenario:

• You were granted Full Access or folder level rights to a room mailbox in another forest.

• You add the room mailbox as a shared Calendar in Outlook.

• You right-click the Calendar folder and then select the Permissions tab:

  

• You select Add, select a user or security group from the People Picker, and then select OK. The user or security group is displayed, and you can select a permission level:

  

• When you select Apply or OK and then return to the Permissions tab, the user or security group is no longer listed:

  

In addition, when you right-click an Outlook folder of the user or room mailbox, and then select Properties to add folder permissions, you receive the following error message:

Cannot display the folder properties. The folder may have been deleted or the server where the folder is stored may be unavailable. Cannot display folder properties.

Cause

This issue occurs if the mailbox account isn't added to the profile correctly. The folder properties will not be accessible.

Resolution

To work around this issue, use one of the following methods:

• Add folder-level permissions to a room mailbox by using the Add-MailboxFolderPermission cmdlet.
• A user in the same forest as the room mailbox can add permissions in Outlook.

If the mailbox is a user or a shared mailbox, add the mailbox as an additional email account. To do this, follow these steps:

1. Select File > Info, and then select Account Settings.
2. Select Account Settings.
3. On the E-mail tab, select New.
4. On the Auto Account Setup page, enter the room or user mailbox name, email address, and your account password.
5. Select Next.

You will then be able to add permissions.