In addition to Yahoo! Inc. (NASDAQ: YHOO) announcing that it will soon offer unlimited email storage to is email customers starting this May, the web company has now announced that it will open up is email platform to external developers who can add utilities, features, and other applications with the world's most popular web-based email service.

It's no surprise that Yahoo! is doing this. By far, Yahoo! Mail is the company's most-used feature among global customers, as it counts more than 250 million customers worldwide (according to the company). By comparison, Google Inc.'s (NASDAQ: GOOG) Gmail claims only 65 million customers after its launch back in 2004 (which was limited, but now it is available to anyone). Both are estimates though -- the companies are kind of tight-lipped on specifics.

Yahoo! wants to encourage developers to change and customize Yahoo! Mail to their liking in order to have more loyal customers. No new strategy here, as many web-based applications do the same thing. There is no way to please the entire customer base with a single offering, so companies let developers (and customers) change base applications to the way that suits their lifestyles and workflow.

Will this release allow Yahoo! to remain the web's most-used email service? It will help, and I don't see Yahoo! Mail giving up that title anytime soon. In this case, Yahoo!'s first-mover advantage in email helped it gain a very large head start in the market, even as Microsoft Corporation (NASDAQ:MSFT) acquired Hotmail to compete better and Google released Gmail about seven years later than Yahoo! released its email product.

Google may be grabbing more headlines these days and may have a stock price that gets the media excited (along with everyone else), but Yahoo! dos have a shining star, and it is the company's email service. It will be quite a while before Yahoo! relinquishes that title.