A Domain Key is an electronic signature that is applied to every email you send to verify your identity and that your email address belongs to the domain used as the originators address.
Domain Keys Identified Mail or DKIM is a very useful anti-spam tool and is virtually impossible for the "spammers" to forge. Therefore it gives a better level of confidence in any anti-spam software or processes that your email is genuine.
DKIM is enabled by default on all accounts to screen both incoming and outgoing mail but you can change this setting within cPanel under the "Email" section by clicking the "Authentication" icon.
We recommend you use this in conjunction with your SPF Record as authentication for your emails for even greater levels of anti-spam confidence.