[exim] Permission denied while creating lockfile

Hello,

on a webserver, where users send email by calling /usr/sbin/sendmail,
I have dozends of entries like this in the mainlog:

2007-06-20 00:45:02 1I0mRa-0003LA-Cm == u15733@myserver R=localuser
T=local_delivery defer (13): Permission denied: creating lock file
hitching post /var/mail/u15733.lock.myserver.46785c6e.00003232
(euid=15733 egid=1000)

What I don't understand is: Why is exim trying to make lockfiles? And
why is he trying to create lockfiles in /var/mail? I have configured a
completely different spool directory.

In fact, the permission is denied, because the users don't have write
access to this directory.

Regards
Marten