Hi Teh,
Currently there are limited available customizations for Dynamics 365 App for Outlook.
e.g: set available entities in app designer, edit sitemap in app designer.
Unlike Dynamics 365 entity form, we couldn't add detection rule for form in the App.
I have tested to set Regarding field of Email entity to business required, however, it'll skip detection to create email record with an empty regarding value.(while obviously it works in Dynamics email form)
A working method is that to create a real-time workflow to detect the field after record is created of before record is updated,
then after we click "Track without regarding", the status will keep in pending status and can't be converted as an email record into system.(Even if we re-open the App again, the email won't be tracked)
But it won't give user any error notification.
So in a word, currently this part couldn't be customized, it's by design.
Regards,
Clofly
