It might be possible you are encountering this error while sending or forwarding certain email messages:
Error in system filter: failed to open filter log file "/var/log/filter.log":
Permission denied (euid=203 egid=203)
This error likely means that your server's Exim filter file is trying to write to the file "/var/log/filter.log" but it has improper permissions, or the file does not exist.
First, perform a "touch" to ensure the file exists:
Then, perform the following to ensure the file is owned by the user and group "cpaneleximfilter":
chown cpaneleximfilter.cpaneleximfilter /var/log/filter.log
Finally, restart the exim service using the following command: