Using older versions of Outlook for Mac in business environments
If you have a hunt around you'll see this is one of many faults with the very, very buggy New Outlook that have never been addressed (large font issues, flaky importation of data etc). The only real solution is to revert to legacy. My wife and I both keep an old Mac with the legacy Outlook on it and use that if we need to search.
If you have managed to import data - it'**** and miss - you may find the missing mail by going to the folder it's in, but the New Outlook seems unable to do a search of all your emails. You get random hits if you are lucky. Rebuilding the index doesn't help.
New Outlook is definitely VERY, VERY buggy.