To make sure that the central docketing user has access to all users mailbox you will need to reach out to your I.T. or have access to a Office 365 global admin account and follow these steps.
1. Sign into portal.office.com
2. Click the 9 dots in the top left hand corner and click admin.
3. Click on User > Active users.
4. Find the User that you would like the Central Docketing User to have mailbox permissions for and click on that user.
5. Click on Mail > Read and Manage Permissions.
6. Then add the Central Docketing user to have access to this users mailbox.