Qmail - using Plesk to add wemail users getting error "no Mailbox found"

Well I have qmail setup and things work well for user that I create
with the unix 'adduser' command they get mail via Pine.

When I use Plesk 7.5.2 to add webmail users I get bounced message that
there is no usewr Mailbox of that name.

Anyone have any suggestions. I have looked at the Plesk site and also
through this group and still nothing. I've even read through "life with
Qmail" several times to no avail.

Any help would be greatly appreciated.