[Exim] exim: permission denied

Hello everybody. I'm new to the list, and this is my first time posting
a question here. Thanks in advance for your patience!
I recently installed woody 2.4 for use as a POP3/SMTP server. I
configured exim and qpopper. the pop side is working like a dream, but
I'm having an intermittent SMTP problem. about 1/6 emails returns me a
mail to the mail admin address with the following (I added the Subject
and Body for clarity):

Subject:
Cron <mail@mail> if [ -x /usr/sbin/exim -a -f /etc/exim/exim.conf ];
then /usr/sbin/exim -q ; fi

Body:
exim: permission denied

the intermittency of it is one of the most frustrating things. I checked
permissions for usr/sbin/exim and I can't see anything wrong with them,
but hey, I'm a noob. I also crossposted this to the Debian-users list,
because it seems like it might be a filesystem/permissions problem.
Can anyone point me in the right direction?
Thanks again,

-Peter