You could try deleting you account from here. It may depend if you are signed on using Microsoft account or local account though - I don't think you can remove the MS account you sign on with.
You can only delete the mail app if you delete the calendar app as well as they are part of one package.
If you want to do that, copy and paste this into an elevated powershell window
(From Apps - Uninstall in Windows 10 - Windows 10 Forums). You can always download it again from the store if you want to.
Get-AppxPackage -Name $app -AllUsers |
Get-AppXProvisionedPackage -Online |
where DisplayName -EQ $app |