Also, it's worth noting having been an E-mail administrator for many companies using Outlook that it's important to know whether your User is using a corporate or business version of Outlook which uses something called
EXCHANGE SERVER, which is a high-performance dedicated E-mail server PC running Microsoft server software specifically for corporate E-mail. If this is the case, most of the free tools mentioned I've tried do not work on any Outlook Client which is pulling down E-mail from an Exchange Server somewhere.
You'll almost certainly need a paid product to fix that. Since you mention you have a User with that problem, and you work for a Company are you in their IT department? If so, have you contacted your E-mail...