Dynamics 365 App for Outlook is not supported for cross-tenant configurations.

Users with an exchange online cross-tenant authentication email profile can’t set the regarding field within outlook client/OWA for Emails and appointments. Mailbox configuration and publishing of the outlook app can be successfully done on CRM end. But the outlook app doesn’t appear in outlook client or OWA.

Only way to set the regarding field within outlook (for Emails/Appointments) is to set the category (Tracked to Dynamics 365) and once it’s synced with CRM then set the category.