Exchange Impersonation enables a caller to impersonate a given account so that the operations can be performed with the rights of the impersonated account, instead of the rights that are associated with the caller's account.
NXPowerLite needs to be run with a user account that is able to impersonate each of the other users in the Exchange Server. Without this permission NXPowerLite will be unable to either scan or replace files on your Exchange Server. Instructions for configuring impersonation are on Microsoft's website.
More information on using impersonation is available here.