Not even a full week after Microsoft let Windows 10 build 10049 loose on the Web, the next build, 10051, has leaked. As you might expect from an OS a mere two iterations higher, there's not a lot new (at least, that I can immediately see) here that wasn't available in 10049. But as Neowin spotted, the Mail and Calendar apps have been updated.
If you've dabbled in any one of the Windows 10 builds released so far, the updated Mail and Calendar apps are going to feel right at home. As you might be able to tell from the shot above, Microsoft is emphasizing minimalism and modernism quite a bit here; there's very little wasted space, and ultimately, the apps are simple and enjoyable to use.
If you look back to the above-linked post, where we posted a couple of screenshots of Microsoft's Spartan Web browser, you can see that Microsoft is keen to make sure all of its apps adhere to similar styling. Like Spartan, the new Mail and Calendar apps have slide-out menu screens - something I'm not quite sold on, as I prefer fixed menus that I don't have to fight with.
There's no telling when the next general preview build will be released, although it seems like Microsoft is trying to space them a month apart (with the exception of 10049, as it introduced a major component, Spartan) so that each new build will introduce a number of enhancements and features, in effect making upgrading worth our time.