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Currently, the only way to prevent entities from appearing in the App for Outlook Set Regarding lookup is to remove them from the Entity Definition of the app itself. This prevents use of that entity in other areas such as the Sitemap. We should have separate configuration of the Regarding lookup from that of entities that may be used in the sitemap.
STATUS DETAILS
Under Review
Ideas Administrator

Thank you for your feedback. This is a great suggestion! We will consider this in our roadmap. 

Sincerely, 

Naitik Shah

PM, Microsoft.