Tag External Emails from NEW external addresses

@Ivan_Wang - I got it working into a self-maintaining sender list using your suggestions. Here are the steps I took:

  1. Schedule an Exchange Shell script to export a CSV List from delivered emails
  2. Schedule another script to read the CSV and log the addresses from step #1 into a database (so I can keep track of what has been added). This script then compares with my database to identify only NEW senders. It takes the NEW senders and puts them into a CSV file, formatted for import.
  3. Schedule and Exchange Shell script to import the CSV file created in #2 using the New-Mailcontact command.
  4. A fourth script to hide these new mail contacts from address books.(to modify attribute msExchHideFromAddressLists

And then an Exchange rule that checks to see if the sender is NOT member of the group that I'm populating. The limitation of this method is that it treats senders as "new" until the scripts run again. I am running them daily and so in the rule disclaimer, I said something like "this is a new sender... etc... This message will stop appearing for this sender after 1 day."

I spent all day this... I don't think the catch was worth the chase. But it is working.