I recently had to remove permissions from a mailbox where another user had “Full Access” or “Full Permissions” over. The original setup was done on the old Exchange 2010 server that was upgraded to 2016.

• Log into the web interface of the Exchange.
• Locate the mailbox and double-click on the mailbox display name.
• Click on “mailbox delegation“.
• Scroll down to the last box “Full Access“.

Let us know if this helped you…