Again, until the preview build drops, a lot of this is speculation at this point, but this article caught my attention:
That being said, there is chatter regarding universal apps being different and actually pre-installed with Windows 10. I'm pretty confident that it won't be the default mail app as its always been part of the Microsoft Office suite as you've mentioned, though you could obtain it separately with the standalone suite packages, that's less likely these days with Microsoft offering Office 365 subscriptions over the old once off cost packages, similar to how Adobe Creative Cloud is over the Creative Suite versions.
Currently Microsoft does not have a public build available that contains the Office touch apps, and the preview for phones build has not yet been released but should be dropping sometime in February.
I'll be getting my hands on a build of Windows 10 Technical Preview and of Windows Phone 10 (Which will likely be branded as just Windows 10) when they are released through the Windows Insider programme. Microsoft Word, enough said really, if its anything like the Desktop currently, oh dear. Happy for consitency across the three platforms and sad because, well.
If it does turn out to be Microsoft Word for rendering email, I am both happy and sad to hear this news. While Windows Central mentions formatting, I would take that as perhaps the rendering engine as well, but its too early to tell at this point. Something which is already in use on the desktop client, but isn't part of the Outlook Mobile client on Windows Phone currently, as it has used IE Mobile for rendering since Windows Phone 7. Not much information on the Outlook client itself currently, but an interesting note in the article was reference to the Microsoft Word engine. It has also been announced that it will be the default email client in Windows 10, replacing the current calendar and email Metro apps in Windows 8/8.1. It will be a universal app across all platforms. Its been revealed that Windows 10 will include a redesigned Outlook email client.