Check the search scope first
Before rebuilding anything, look at the search bar. Outlook can search only the current folder, the current mailbox, subfolders, or all mailboxes. If you are sitting in the Inbox and the email you want was filed into a client folder last year, a current-folder search will never find it. Switch the scope to all mailboxes and try again.
The same goes for shared mailboxes and archives, which often need to be included explicitly. Also try the same search in Outlook on the web. It searches the server copy of your mailbox, not anything on your computer. If the web finds the email and the desktop app does not, the problem is local to that computer, which narrows things down a lot.
Cached Exchange Mode and the download window
Classic Outlook on Windows keeps a local copy of your mailbox so it works quickly and offline. To save disk space, it is often set to download only the last few months or a year of mail. Older items are still on the server, but search only looks locally unless you click the option to include more results from the server, which many people never notice at the bottom of the results.
Go to Account Settings, open your account and look at the mail to keep offline slider. Extending it to cover all mail makes older items searchable locally but uses more disk space and takes time to download. On a laptop with a small drive, leaving the window shorter and using server search for older mail may be the better compromise.
Rebuilding the search index on Windows
Classic Outlook on Windows relies on the Windows search index. If that index is corrupted or stuck, search returns partial results or nothing at all. Check indexing status from the search tools menu in Outlook; if it says items are still being indexed days later, the index is likely stuck. Open Indexing Options in Windows, choose Advanced and rebuild. Leave the computer on and Outlook open overnight while it works through the mailbox. Also check that Outlook is included in the indexed locations. Group policy tweaks, aggressive clean-up tools and some antivirus products remove it. If rebuilding does not help, a fresh Outlook profile often does, because the local data file is recreated from the server rather than repaired.
New Outlook, Mac and very large mailboxes
The new Outlook for Windows behaves more like the web version and searches the server, so indexing problems mostly disappear, but it can be slower on very large mailboxes and offline search is more limited. Outlook for Mac uses Spotlight, so if Spotlight has been told to ignore the Outlook data folder, search breaks. Removing that exclusion and letting Spotlight re-index usually fixes it. Mailboxes with hundreds of thousands of items strain every version. If one person's search is consistently slow, look at whether their mailbox has grown into a dumping ground. Moving older mail into the online archive, or clearing out large folders of automated notifications, helps search more than any setting.
Search tricks that save time
Outlook understands simple search keywords. Typing from:aroha finds mail from that person; hasattachments:yes narrows to emails with files; received:last week limits the date range. Combining them, such as from:accounts subject:invoice received:this month, gets you to a specific email far faster than scrolling. The search ribbon offers the same filters with buttons if staff prefer clicking.
If several people in the office report search problems at the same time, it is less likely to be individual indexes and more likely to be a Microsoft service issue or an update. Check the service health page in the admin centre before spending time rebuilding indexes on every computer.
When search problems point to something bigger
Occasionally a search failure is a symptom rather than the problem. A local data file that keeps corrupting can mean a failing hard drive, especially on older laptops with spinning disks. Outlook constantly showing it is updating folders, or crashing during searches, can point to a damaged profile, a faulty add-in or a computer that is simply short of memory for the size of mailbox it is holding.
Try starting Outlook in safe mode, which disables add-ins. If search and stability improve, re-enable add-ins one at a time to find the culprit; old PDF tools, CRM connectors and meeting plug-ins are frequent offenders. If the computer is several years old and struggling generally, replacing it may save more staff time than continued troubleshooting. Keep a note of what you tried, so the next person working on the computer does not repeat the same steps.
In short
- Check the search scope and try Outlook on the web before fixing anything.
- Cached mode may only keep recent mail on the computer.
- A stuck Windows search index is the most common desktop cause; rebuild it overnight.
- Huge mailboxes make every version slow; archive old mail.