An Office 365 user tries to open another user's Microsoft Outlook mailbox folder to which the user was granted access. However, the mailbox folder doesn't open. Instead, the user receives the following error message:
Unable to open default folders-you do not have permissions to logon
This issue occurs if the user doesn't have sufficient permissions to access the mailbox folder.
When a user wants to give another person access to one of his or her own mailbox folders in Outlook, the user has to grant the other person access to the specific folder that the user wants to share and also to all other parent folders in the folder hierarchy. It's not enough for the user to grant permissions only to the specific folder that he or she wants to share.
For example, user B wants to grant user A access to the Invoices folder, and the Invoices folder is a subfolder of the user's Inbox folder. To grant this access, user B has to grant permissions to user A for the following folders in Outlook:
The Invoices subfolder
The Inbox folder
The parent Mailbox folder
For more information about how to set up mailbox permissions in Outlook, go to the following Microsoft website:
Note The root folder (the Mailbox folder) in Outlook must be assigned at least "Folder visible" permissions for the default role.
In Office 365, Exchange Online admins can also grant mailbox folder permissions by using Exchange Online PowerShell. The Add-MailboxFolderPermission cmdlet can be used to grant access to specific folders in a user's mailbox. For more information about how to use this cmdlet, go to the following Microsoft TechNet website: