Microsoft Knowledge Base Article 246151 contains the following summary:

To gain access to the Messaging Application Programming Interface (MAPI) e-mail client profile, you must configure the Microsoft Windows 2000 Fax Service to run under an Administrator account.

To enable MAPI functionality in the Fax service, you must configure several items:

The user must be a member of the local computer Administrator group and you must install a MAPI-enabled e-mail client (for example, Microsoft Outlook or Microsoft Exchange Client).
You must have a MAPI profile for the user account.
You must change the Fax service from the default system account to the e-mail client user's account.
This action changes the Received Faxes box in the Windows 2000 Fax Management console modem properties to send the received faxes to local e-mail inbox using the profile name from the e-mail client.