When Outlook search comes back empty for an email you know exists, the mail is almost never gone. Either search is looking in the wrong place, it’s only searching recent mail, or the index that powers it has quietly broken. In that order of likelihood:
Check where search is looking
Click into the search box and check the dropdown beside it. Current folder won’t find mail that was filed somewhere else – switch to Current mailbox or All mailboxes and try again. And if you have shared mailboxes attached, make sure you’re searching the account the mail actually lives in.
Mail older than about a year
Classic Outlook usually keeps only the last 12 months on your machine; everything older stays on the server, and a default search may not look there. Scroll to the bottom of the results and click the link to search the server for older mail. To fix it permanently: File > Account Settings > Account Settings, double-click your account, and drag the Download email for the past slider towards All – your mail was on the server the whole time, this just keeps a full local copy.
Rebuild the search index
Classic Outlook on Windows leans on the Windows search index, and it does occasionally corrupt:
- Go to File > Options > Search > Indexing Options
- Click Advanced, then Rebuild
- Leave the machine on and plugged in – a big mailbox takes hours to re-index, so end of the working day is the ideal time. Search stays patchy until it finishes.
The 30-second sanity check
Run the same search in Outlook on the web at outlook.office.com – the web searches the server directly, with no local index to break. If the mail shows up there, it exists and your desktop index is the problem: rebuild it as above. If the web can’t find it either, check Deleted Items and any archive folder – and if you think it was deleted a while ago, raise a ticket quickly, because deleted mail is only recoverable from the admin side for a limited window.