Outlook 2016 additional mailbox not updating
Either way, you should be able to do this without knowing the password for that mailbox.
Note 2: Adding a mailbox as an additional Exchange account is only possible in Outlook 2010, Outlook 2013 and Outlook 2016.
So, even if you’re using the new Outlook 2016, you will eventually run into some issues.
One of the most common issues that occur with most Outlook clients is the search tool not working as it should.
However, your Exchange administrator must have granted “Full Access” permissions for that mailbox to your account for you to be able to add it as an Exchange account.