Here's a quote from the reported email:
I understand that you are hearing concerns from certain partners about Microsoft's commitment to the mobile space.
Let me be very clear: We are committed to deliver Windows 10 on mobile devices with small screen running ARM processors.
We are currently in development of our next generation products and I wanted to reconfirm our commitment to Windows 10 Mobile. We believe in this product's value to business customers and it is our intention to support the Windows 10 Mobile platform for many years. We have a device roadmap to support that from Microsoft as well as our OEM partners who will also be selling an expanded lineup of phone devices based on this platform.
There's an important clarification here, of course - "to business customers" - there's a recognition that the core USP of Windows 10, the cross-form-factor nature, evidenced in the likes of Continuum, will be the focus in the future. And this is worlds away from consumers checking Facebook and playing games on their everyday 'phones'. In other words, this is another acknowledgment that the battle for 'consumer' mind share is over (in raw sales terms it wasn't much of a battle), versus Android and iOS. But it does emphasise that professionals like me and you, reading this, enthusiasts, business customers, aren't going to see Microsoft give up on its 'One Windows, everywhere' vision anytime soon. And that does, yes, include mobile, arguably the most important form factor of them all over the next decade.