This error means the Exim mail server process is running as user 47 and group 12 (probably exim:mail). It's trying to read the file /etc/backupmxhosts as part of its delivery process, but the permissions of that file are such that the user:group (exim:mail) cannot read it, or it could be a symlink to a non-existent file. Fix your ownership and/or permissions of that file and that will resolve the issue.