Find Contacts with UDFs "in this item"


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Outlook 2019 32-bit
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Is there a way, either manually (using the Outlook UI) or programatically (via VBA), to ask the question "which Contacts have any UDFs in this item"?

I'm using Outlook 2016. I have about 20k Contacts, which came from multiple sources (like ACT). I want to find the Contacts that have "UDFs in this item" as a way of knowing what the source was, and then taking some action based on that. For example, a Contact that came from ACT will have a UDF named "ACTContactID".