When you upgrade to Windows 10 and select to log in with a Microsoft Account you will automatically have your linked email account created and available in the Mail app. The bonus is that any account in the Mail app on Windows 10 is also created in the separate Universal Windows Platform Calendar app.
If you choose to use a local account to log into Windows 10, which Microsoft does allow, you will then need to setup your email account in the Mail app which, as mentioned above, will also add that account to the Calendar app.
This process will also work for adding additional email accounts to the Mail and Calendar apps. Here's what to do.
Bonus history lesson: Windows 10's Universal Windows Platform Mail restores some features that will seem familiar to anyone who used Windows Live Mail in Windows 7.
The Windows Live Mail program, free on the system, was quite robust and allowed you to manage email easily. Unfortunately, the Modern/Store Mail app which superseded it in Windows 8/8.1 was not quite as full-featured as Live Mail was. For example, users could not longer use rules to sort and store their email.
This Windows 10 iteration of the email program, the Universal Windows Platform Mail, gains back some of the robustness that was lost with the Windows 8/8.1 Mail app, and allows users greater control over how they store mail.