Outlook on the Web shows you your mailbox in real-time, so searches should return exact results. There is a limit to how many results will be returned in a single search. It is important to be specific when using the search function. Microsoft has provided documentation with helpful tips as to how to form your searches to efficiently find messages.
Microsoft offers documentation and guides for fixing search issues on both of their Outlook Desktop clients.
Troubleshooting Outlook Search Issues for Windows