Rebuild Reverse DNS Cache and Update Mail HELO
This setting updates the reverse DNS cache and user domains for mail HELO. This setting only appears when you enable the Use the reverse DNS entry for the mail HELO/EHLO if available setting.
Above solution is is the best way when you do not set PTR records prior to installing cPanel. cPanel will detect wrong PTR records due to which Exim mail server will not function. Setting proper PTR records is mandatory. Make sure CNAME is also set in domain DNS settings which is by default used by outlook and other mail clients.