Currently within the Dynamics 365 for Outlook add-in, the configuration wizard allows users to take advantage of functionality to associate a different email account with their Dynamics 365 user account (not just the email that is associated by default by Office 365).

This functionality is missing from within the new lightweight Dynamics 365 App for Outlook.

The add in can currently work as follows: Using the Outlook add-in I would be able to associate my Tenant1 email address (user@tenant1.com) to a D365 instance in another tenant (Tenant2). This would mean that I could track emails from my Tenant1 Outlook mailbox into the D365 insatnce in Tenant2 by changing the primary email address and associated mailbox information on my Tenant2 account in D365. This functionality is not available in the new lightweight app and with the deprecation of the add in this functionality will be lost from D365.