Take a directory of the file so you know it's permissions and ownership.
Then delete it.
Then recreate it:
# touch mainlog
then change the permissions and ownership as necessary with chmod and chown.
I'm not sure if mainlog is managed by syslogd or by exim. So to be safe restart both:
# service syslogd restart
# service exim restart
(The above two commands work for CentOS and RHEL. For other OS distributions, restart the two daemons as specified for your distribution.)
Jeff