[Exim] mailbox has wrong uid

Hello all,

    I'm attempting to deliver all mail to thisdomain.com to a user
called "user1". I have created a transport like this:

 this_transport:
           driver = appendfile
           file = /var/mail/user1
           envelope_to_add
           return_path_add
           delivery_date_add
           user = mail

and I have a special router like this:
 this_router:
           driver = domainlist
           transport = this_transport
           route_list = thisdomain.com

So, this all works, and exim attempts to deliver mail to said mailbox, but
it has the following problem in the logs (and in a message back to me):

2002-05-25 13:00:36 17BeuC-0008IT-00 == yuck@???
T=this_transport defer (-6): mailbox /var/mail/user1 has wrong uid (1006
!= 8)

8 being the uid of the "mail" user, 1006 being the uid of "user1". How do
I solve this problem? Why does it care that it's the wrong uid? How does
this normally work? If I change it to deliver it as the user "user1", then
it has a permission denied error when trying to create the lock file
hitching post.

This is exim 3.35-1 on debian woody.

Please reply to me explicity as I am not on the list.
Thanks in advance!
    -Josiah