umask for saved emails

I was wondering how you would change the umask that sa-exim saves the
files in /var/spool/sa-exim/SApermreject and the like.

I was planning on having these folders set up so they could be viewed
by courier imap by a select group of people. I've set them up setgid to
enable that, however the mail are delivered 600, is there any way I
could change it to 660?