We were facing the following mentioned error for emails on the server and the emails were discarded with following message.
Error in system filter: failed to open filter log file “/var/log/filter.log”: Permission denied (euid=506 egid=559)
The Above mentioned error is basically due to improper ownership assigned to the file “/var/log/filter.log” .
To sort the error you can do the following:
1) Check the ownership assigned for the file
ls -l /var/log/filter.log
2) The ownership of this file must be cpaneleximfilter instead of root.
3) change the ownership of the file as follows:
chown cpaneleximfilter.cpaneleximfilter /var/log/filter.log
4)Now just restart the Exim service on the server. This should resolve the error.