Email Notification on Order (user enters email address on billing page).

Company settings > Email tab >- choose 'Settings' from the dropdown then in the Email Notification at Billing section there is Manual Email Notification -  select ‘Optional’ or  'Required' from the dropdown.

You can change the ‘Email to Notify’ text that appears on the billing page. Go to More> Labels and in the Billing Page Labels section there is an ‘Email to Notify’ field.