I'd like to throw my hat into the ring here and mention a few specific features that you may be interested in around the topic of discounts via email.
Rely upon Magento core auto-generated coupon codes - Some extensions sort of work around the core auto generated coupon code functionality and this can introduce unnecessary complexities.
Include the coupon code in the email - In addition to a link that magically associates the discount to the shopping cart, it's good to have the coupon included in the email as well as a fall back. That way if they want to place an order over the phone they have the coupon and can give it to the customer service rep.
Magically apply the coupon code - While the manual coupon code is a nice fallback, you want to automatically apply the discount as well, which is the main feature that you're asking about here.
Remind the customer they've unlocked a coupon - Even if the coupon has been applied to their cart, it may not be obvious to them that that has happened, so yo