Software, deployment and product key

If you are a indie developer. Specially if you are still a desktop application developer. You might have come to this very point when deploying your product. Yes you always wanted to put a CD key or product key with that application. But you are smart enough to know that keeping a list of verified keys on the desktop application would be stupid. And for whatever reason it is you can't afford a server. And there's hardly any easy to use service to maintain the used key list. Well here's something that I did when deploying my super secret application that I never talk about. Email server. Yeah you heard me right. So I opened a new email account for free for the application. I put a pop3 or smtp server on the application. Now if you are smart you already figured out what I did next. But sure I'll tell you. Since it's a desktop application. I setup my new email account on the application. So whenever the user inputs the product key for verification all I do is download all the mails from my new account and check for subjects that matches the product key :p . If I find match I delete the mail and activate the product. Whenever I need to create a new product key I send a mail to that account with the product key as subject. Surely you can put more security in this system by checking email sender and putting content on body. This method is secure as long as nobody knows what email is embedded in the system. It's also cheap as you don't have to maintain an API server or something. You can do this for games as well or Android apps.