How to Search Outlook
You should see this at the top of your screen, then click the little drop down arrow next to the magnifying glass,
and choose Search Options.
Then choose to search “All folders”
Then back in the normal screen, enter some word(s) to search for in the search box, and hit the Enter key to tell it to search.
It may take awhile to run your search, so just let it do it's thing for awhile.