Exchange 2003 problems with Virtual Domains.

I have setup exchange 2003 running on an win 2003 server (same box)
and have set it up to host mailboxes for several domains.

I?m getting name space collisions when adding users with the same user
logon to separate virtual domains.  I mean I can add
webmaster@test1.com and also webmaster@test2.com okay.  But the
problem arises when I come to the User logon name (pre-windows 2000)
box.  It has a fixed (grayed out) netbios domain name (CU\ in this
case) for each of my virtual domains and so it warns the user already
exists.

Help!

Thanks in advance,