If you are referring to the mail application, it is part of the OS and can not be removed. It can be removed from the start screen by long pressing on it and tapping the bubble in the top right. It will however remain in the app tray to the right of the start screen.
If you are referring to the Outlook account, it can not be removed if it is set as your Microsoft Account. The only way to get rid of it is to restore the phone and sign into another Microsoft Account from the get go.