Despite a lack of any official word, it looks like Outlook.com Premium has exited preview. As reported by Thurrott.com, the premium version of the web email appears to have dropped the preview tag that it's carried since it started testing last year, pointing towards its general availability.
For those unfamiliar, Outlook.com Premium is, well, a premium version of the web-based email service. Microsoft is currently charging $19.95 per year for the service, though it will eventually shoot up to its regular price of $49.95 at the end of March. For the money, Outlook.com Premium provides a personalized email domain that you can share with up to 5 other people. Premium also removed banner ads from your Outlook.com mailbox.
At the moment, Outlook.com Premium appears to still be U.S. only, so you'll have to wait for access if you live elsewhere in the world. For now, you can learn more about Outlook.com Premium by heading to its website and checking out the service's FAQ.