Expand the fields search on in Regarding lookup
To have Dynamics 365 App for Outlook Regarding lookup search on custom fields, that field must be added to the entity’s quick find view.
In Customer Engagement, go to Settings > Customizations > Customize the System.
Choose an entity, then choose Views.
Open Account Quick View.
Enable the custom field, and then choose OK.
Choose Save > Publish on the Home tab.