You also need to check the permissions of every directory up the tree. A permissions problem on a parent directory could cause permission on /var/log/exim/mainlog to be denied.
The permissions and ownerships on the file and its parent directories should be like this:
Code:
# ls -alh /var/log/exim_mainlog
-rw-r----- 1 mailnull mail 197K Apr 21 08:33 /var/log/exim_mainlog
# ls -alh /var/
total 198K
drwxr-xr-x 27 root root 4.0K Apr 11 11:55 ./
drwxr-xr-x 27 root root 4.0K Apr 20 21:53 ../
...
...
drwxr-xr-x 11 root root 4.0K Apr 21 04:06 log/
# ls -alh /
total 314K
drwxr-xr-x 27 root root 4.0K Apr 20 21:53 ./
drwxr-xr-x 27 root root 4.0K Apr 20 21:53 ../
...
...
drwxr-xr-x 27 root root 4.0K Apr 11 11:55 var/
If you see any difference in the permissions or ownerships on your server compared to what I just gave you, you need to change them.