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:
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Start Outlook.
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Click File > Account Settings > Account Settings.
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On the Email tab, select your email account and click Change.
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Drag the Download email for the past: slider to the desired amount of time or to the far right to All.
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Click Next, and then Done. Close out the window and restart Outlook.
