Go to Settings by clicking the gear icon in the top right corner:

This will load the "Quick Settings" panel on the right.  Now, scroll to the bottom and click "View full settings" (it may not be obvious that you can scroll down in that panel)
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Next, a full-screen dialog box will appear.  Click on "Junk Email".

In the Junk Email configuration section on the right, scroll down until you see the "Safe Sender" section.  Then just add "mg.goodshuffle.com" and "goodshuffle.com" and click the blue "Add" button.

That's it! You're all set.

Your menu options to get to your account settings may look like the image below.  Click "Options".
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Next, click "Safe Senders" on the left and you will see the same input fields to add "mg.goodshuffle.com" and "goodshuffle.com" to the safe sender list.  Be sure to click the + "plus" icon for each value and click "Save" to ensure your settings are saved.