Go to this link; https://www.godaddy.com/gdshop/products/smtp_relay.asp
Add that to your cart,
Then add the domain you wish to buy,
Go to your cart and ptu in the promotional code 'david',
Purchase your domain, then delete off the SMPT relaying from in your GoDaddy control panel.

This offer will stop working tonight, but it probably won't be too long before something similar is back .