[Exim] exim -q "exim: permission denied" problem

I have exim installed as:

-rwsr-xr-x    1 mail     mail       525804 Dec 27 22:47 /usr/sbin/exim

and a cron job running exim -q every 15 minutes as the user mail. Run
as mail, there is an error "exim: permission denied". Run as root there
is no error. All of the files in /var/spool/exim are mail:mail

What could be causing this problem?

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