Block All Domains Whitelist Select Domains

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http://forums.cpanel.net – In cPanel how do you set up filtering to BLOCK all domains for both inbound and outbound emails and whitelist my company domains. I am looking to only allow inbound and outbound emails to be sent from 4 company domains and block employees from receiving and sending emails from all other domains. Emails are to be used for business purposes only and therefore all non-business domains need to be blocked. Thanks everyone. (HowTos)