I'm not sure if this is specific to 2016, which I recently upgraded to. So in my indexing options Outlook is there. NowI switch it off and save and the number of indexed items drops by a thousand or so. Close and reopen outlook just for good measure. Try a few "from:xxx" searches - the first one displays a message at the top of the results that the search couldn't be completed but it looks like results were returned anyway. Subsequent searches appeared to work normally. Sometimes I get the message again, but it still works.
Then re-enable Outlook in indexing options, open outlook and wait for indexing to say its finished. Now try the searches again. I don't get any warning messages time, but I also get the same results as when indexing was off and the speed of the results is about the same (extremely fast).
So other than preventing the...