DKIM (DomainKeys Identified Mail) is a means of verifying incoming email, and SPF (Sender Policy Framework) system allows you to specify servers and IP addresses that are authorized to send mail from your domain(s). These features are called Email Authentication in cPanel interface, from where you can activate these features. Here's a tutorial that explains how to use these features: http://webx.net/dkim-spf-email-authentication-cpanel/
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