RE: D365 App for Outlook - Field grayed out
Yes - it is grayed out in the UI as well (we're on-prem and still using classic web, which is why I hadn't noticed that little tidbit!).
Here's an active contact showing the field filled in. It's locked, and there's a little megaphone next to it.
There are no active business rules associated with this field.
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Again, this is the only lookup field that's grayed out!
Not sure how I would check if there's any javascript associated with this field.