The symptom
Auto-Complete stops suggesting names or addresses you have emailed before. New entries never appear. The feature relies on a local cache that can corrupt after updates or profile changes.
Exchange accounts
Close Outlook. Turn off Cached Exchange Mode in Account Settings. Navigate to %LOCALAPPDATA%\Microsoft\Outlook and rename the RoamCache folder to old_RoamCache. Restart Outlook, re-enable Cached Exchange Mode, then restart again. The next sent message repopulates the list.
Other account types (IMAP, POP, Outlook.com)
Close Outlook. Open %LOCALAPPDATA%\Microsoft\Outlook and rename RoamCache to old_RoamCache. Start Outlook. The cache rebuilds on the first send.
What to expect
The old list is gone. New recipients populate normally after the first message. If the problem recurs, the Outlook profile itself may need rebuilding.