Hi,
Maybe you can try to remove the delegate access/full access permissions for the user from the mailbox using Windows PowerShell. You can do this with following command:
Remove-MailboxPermission -Identity <Mailbox ID1> -User <Mailbox ID2> -AccessRights FullAccess
Note, in this command, <Mailbox ID1> represents the mailbox to which the user is granted permissions, and <Mailbox ID 2> is the mailbox of the user from whom you want to remove full access permissions.
For how to Connect to Exchange Online Using Remote PowerShell, please refer to this
article.
By the way, this is the forum to discuss questions and feedback for Microsoft Office client. For your question, you might want to post in the dedicated forum of
Office 365 Community, where you can get more experienced responses:
http://community.office365.com/en-us/f/default.aspx
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thank you for your understanding.
Regards,
Ethan Hua
TechNet Community Support
It's recommended to download and install
Configuration Analyzer Tool (OffCAT), which is developed by Microsoft Support teams. Once the tool is installed, you can run it at any time to scan for hundreds of known issues in Office
programs.