How do coupon codes work?
The coupon codes use Magento's auto-generated coupon code feature. So there is a shopping cart price rule that gets associated to the email trigger, and then the email trigger generates as many coupon codes as it needs to.
For simple coupons, such as 5% off, we'll create them over the API and you don't have to worry about it. For coupons that are more complicated (requiring conditions, etc.), you can create them in the Magento backend.
Once a shopping cart price rule is created, it just needs to be associated to an email trigger in the Discounts tab:
MageMail will then continually automatically create coupon codes against that shopping cart price rule in batches of 1000 at a time.