Microsoft announced this morning that the next version of the Windows Phone operating system, code-named Mango, has been delivered to manufacturers, which can begin testing it on their handsets. The move is one of the final steps before the software arrives on new phones and is delivered to existing users as a software update. \"This marks the point in the development process where we hand code to our handset and mobile-operator partners to optimize Mango for their specific phone and network configurations,\" Terry Myerson, Microsoft\'s corporate vice president of engineering for Windows phone, wrote on a company blog. \"Here on the Windows Phone team, we now turn to preparing for the update process.\" The update, which brings multitasking and a complete overhaul of the built-in Internet Explorer Web browser, was first discussed in February, at Mobile World Congress. Two months later, Microsoft took the wraps off all the planned features, promising to deliver it in the fall. Mango is the first major system software update to hit Microsoft\'s Windows Phone 7 platform since an interim release near the end of March that added copy-and-paste functionality, improved marketplace search, and enabled faster app loading. Microsoft is not counting Mango as an all-out new version of the platform, something the company has not yet discussed. Microsoft started dishing out a beta version of Mango to mobile developers at the end of last month so that they could test their applications for compatibility with the new features and back-end changes. Update at 04:09 P.M. PT: If you\'re looking for a full rundown of Mango features, you should check out the detailed hands-on of a pre-release build from my CNET colleagues Bonnie Cha and Jessica Dolcourt.