You would begin by accessing your cPanel account following these instructions.

Once in cPanel, you would then scroll to the "Email" section and click on "Spam Filters".

Scroll to the bottom of the "Spam Filters" page and click "Show Additional Configurations".

Click the appropriate link to edit Spam Whitelist or Blacklist.

On the corresponding page for Whitelist or Blacklist, simply click the "Add a New (Whitelist/Blacklist) Item" link, enter the email address and click the Update button. To whitelist an entire domain, enter the wildcard character and the domain (example: *