Windows Mail

Windows Mail is the default mail application in Windows Vista, Windows 8/8.1, and Windows 10. The application was included in some early builds of Windows 7, but in later builds was intentionally broken. While not included in the RTM, remnants of its files can be found in various places including the registry. By copying the program folder from Vista, you can restore Windows Mail functionality in Windows 7.

Longhorn/Vista
Windows Mail was originally introduced as Outlook Express 7.0 in the pre-reset builds of Windows Vista known as Longhorn, most notably build 4074. This version of Outlook featured a new icon for the program depicting an open white envelope with red and blue dots around the spot where the letter would go in. After Longhorn was reset and rechristened as Windows Vista, Outlook Express 7.0 was removed at the same time WinFS was removed, and was downgraded back to 6.0, as seen in RTM Windows XP. The icon was also reverted to the old mail icon as well. Eventually, Windows Mail was included and was the application that RTMed with Vista.

Windows 7
Windows Mail was included in early beta builds of Windows 7. Some could run it, while others had it intentionally broken. This was eventually removed altogether, and was replaced by Windows Live Mail, which had to be downloaded. This marks the first time since Windows 95 that a mail application was not bundled with Windows (every version from Windows 98 to Vista, and then again from Windows 8 onward had one). Leftover files from Windows Mail are still found in Windows 7 RTM, notably in the Registry and the initially hidden AppData folder in the User folder. Copying the program install from Vista leads to Windows Mail working again with a minor glitch with the version string (that is displayed as  instead of a real version).

Windows 8/8.1
Windows Mail was brought back in Windows 8/8.1, but in Metro language form. Windows Live Mail could also be installed on 8/8.1 as well.

Windows 10
The mail app had some changes, but mostly follows the same metro language design as its 8/8.1 counterpart. Windows Live Mail was also available for 10 as well.