Running Outlook 2016 under Win10, working offline.

I created a simple "Run Now" rule to scan a folder containing 2005 messages and move any message with a certain recipient to a temporary folder. I know there are 250 messages with that recipient, but my rule finds only two of them.

This effort is about as simple as you can get, so I'm doing something very dumb -- I just don't know what it is.

Any ideas?