You can increase the time in your email account's offline settings so that more or all your emails are cached locally for indexing. To do this:

  1. Start Outlook.

  2. Click File > Account Settings > Account Settings.

  3. On the Email tab, select your email account and click Change.

  4. Drag the Download email for the past: slider to the desired amount of time or to the far right to All.

  5. Click Next, and then Done. Close out the window and restart Outlook.