Start with the right checks.
Searching the wrong mailbox or folder can look like an indexing failure. Begin with a message you can still locate manually.
What to do next.
Confirm the correct account and search location.
Search a distinctive sender or subject word and clear unnecessary date or attachment filters.
Compare the result in official webmail if available.
If desktop search alone fails, record the Outlook version and use Microsoft’s matching search troubleshooting guidance.
Check the result.
Repeat the search for the known message and another older example. Confirm that the intended mailbox is included.
Stop and get help if…
A rebuild or profile replacement is being proposed, local archives are involved or results differ unexpectedly between devices. Get guidance before making changes to the mail data.