OS Email being redirected externally.

Red Hat Enterprise Linux Server release 6.9 (Santiago)

My client has several servers, that are sending any mail sent to an OS user, such as oracle or root, to the external @ email address and not to /var/spool/mail/, eg. root@ for failed cron jobs.
The client would like us to stop the systems sending these emails, as their email server is getting flooded with emails to invalid users. I have checked for ~/ .forward files and the entries in /etc/aliases and the mail is not being redirected by either of those methods. I assume this was originally setup to prevent /var/spool/mail from filling up.

Where is this "feature" will have been configured?