If you think that Outlook hasn’t indexed all your items, i.e., upon performing a search, you get a result saying that the item wasn’t found, then you can troubleshoot the indexing function. To do that, open Outlook and click on the Search Box. Click on the Search Tools drop-down here, and further click on Indexing options. If all items have been indexed properly, you’ll see the message “Outlook has finished indexing all of your items. 0 items remaining to be indexed.” pop up. If not, wait for a few minutes for the unindexed items to get registered in the index.