From

User ID of the sender (e.g., MILLER)

With the corresponding rights set in the User Profile, a user can select any user ID as a filter, as long as he/she and the user specified belong to the same group or if he/she is the specified user's representative.

Users authorized to access Outbox All Users (as set in the User Profile) can specify any user ID or leave this field blank to see and/or open all users’ messages.