exim spool directory permissions

I just upgraded to exim via RPM to version 4.92.3 on a CentOS 6.10 server, and I can't deliver messages due to exim being unable to write to the /var/spool/exim directories. The directories are owned by user 'mailnull'. I changed the owner of these directories to 'exim', and it seems to help, but something changes the directories back to mailnull ownership.