I came across an issue in Microsoft Outlook 2016 when a user is giving full access to another/shared mailbox it does not display the emails under the mail account.
The first impression is that the permissions were not applied properly but one of the beauty of Microsoft Exchange 2013/2016, the mailbox will be loaded automatically to the user’s outlook profile when the rights are granted.
To correct this problem you will go the following setting in the Outlook client:

Select File menu

Select Info –> Account Settings –>Account Settings

Select Email tab –> Change

Select button More Settings

uncheck option Download shared folders under Cached Exchange Mode Settings.
OK everything and then restart Outlook for the changes to take effect.
I hope this was helpful.