If you see this error in your /var/log/mainlog, then your /dev/null is likely not set with the correct permissions. See this code:
[root@server exim]# ls -la /dev/null crw------- 1 root root 1, 3 Sep 21 05:16 /dev/null [root@server exim]# chmod 666 /dev/null [root@server exim]# ls -la /dev/null crw-rw-rw- 1 root root 1, 3 Sep 21 05:16 /dev/null
so it should be chmod 666, not 600.