Collin Fletcher of The Tech Block makes the solid case that the acquisition of iOS email app Mailbox by Dropbox signals the latter company’s push to build a “windowed ecosystem” on top of its core file syncing service. It’s worth reading the whole article, but I’d go a step further.

Victoria Barret of Forbes reported in October 2011 that, as Dropbox CEO Drew Houston told her in an interview, Steve Jobs called the company a “feature, not a product” when Houston rebuffed Jobs’ acquisition offer.

Steve was wrong, though. Dropbox isn’t a just product (although plenty of people pay for its core sync functionality), nor is it merely a feature (although it can be integrated into many different types of apps and services).

Dropbox is, at its most fundamental level, an infrastructure. What the Mailbox acquisition really signals is that Dropbox is confident enough in its core service that it is time to invest in ways to add value to that service. The Dropbox file storage and syncing infrastructure already undergirds the daily workflows of millions of people.

Email is the perfect product to build on that infrastructure. So is a personal music storage and player product. So is a task manager. So are innumerable other products that benefit from a powerful, reliable file synchronization mechanism that can functions flawlessly at scale.