After granting the permissions as you normally would, giving it a moment and relaunching Outlook, we couldn't expand the mailbox and received the error "Cannot expand the folder".
I rechecked everything and the user had full permissions according to the 365 Admin portal.
I then decided to fire up powershell and see what it said, and to my surprise - it too said the user had full access to the other users mailbox!
Yeah, I thought so too.
Not to waste having logged into Powershell I removed and added the permissions back with the commands below, checked it was set in Powershell and 365 Admin portal - all looked good so fired up Outlook again - HUZZAH! I could now open and browse the mailbox!!!
So to remove and add full permissions use the commands below:
- Remove-MailboxPermission -Identity [email address or alias of user whose mailbox needs no longer to be shared] -User [email of user that needs access revoking] -AccessRights FullAccess -InheritanceType All
- Add-MailboxPermission -Identity [email of user whose mailbox needs sharing] -User [email of user that needs access] -AccessRights FullAccess -InheritanceType All