Forwarding domain emails is beneficial when wanting to simplify the mail between the domains.
It means that rather than each domain being managed independently, you will only have to manage the primary domain for emails.
If the domain is using SpamExperts as their mail exchanger then it needs to be done in the SpamExperts system before the domain is parked/added to a cPanel.
To do this, Log into the SpamExperts server. You can do this via the cPanel API or directly by visiting control.server-filter.com
Once logged into the filter. Click Domain aliases and enter the secondary domain
Once added, you can then add the domain as a parked or addon domain and the domain alias will be in effect.