postmaster can not login?

[lix]

hello all,
        I install qmailadmin with vpopmail.But I can not login with the postmaster.But 
I can login with normal user.
        why? I can see the postmaster directory in ~vpopmail/domains/virtualdomain.com/
        but in vpasswd file there are no entry for postmaster.
        
        Who can give me any advice?