Just received this in the mail. The code is non-targeted and is able to be used by anyone. It is also able to be used by existing customers on new subscriptions (ie, you take on a service you don't already have).
Please read below for discounted services and exclusions - There is no listed expiry but it does say 'December Deal' so I'm assuming it ends on the 31/12/14:
Use the coupon code DecDeal50 at checkout and receive 50% off any new subscription to Managed DNS Express, Email Delivery Express, Standard DNS, or Remote Access.
If you’ve been looking to try out any other Dyn product, now is the time!
Managed DNS Express: Connect visitors to your websites and applications with the fastest, most secure, and reliable DNS.
Email Delivery Express: Get industry-leading inbox deliverability with Dyn’s reliable, easy-to-integrate cloud-based SMTP relay.
Standard DNS: Manage your DNS with a simple solution for your personal or small business website domain.
Remote Access: Access your device or files using a unique hostname instead of a confusing (and ever-changing) IP address.
Just put through an order today for 5 years, 8/1/2015. Code still good.
Note if you went to their home page, they're offering a 28% discount, so this 50% off is a better deal.