How to add Email field in Checkout page and sending E-mail in Drupal Commerce?
I believe you are using Customer Profile for asking customer billing and shipping address. 


So navigate to 
Store->Customer profiles->Profile types->Billing/Shipping Information->Manage fields.
Add one more fields in there for email &amp; make that mandatory.
Add a checkout rules, put condition like "Entity has field" &amp; check your added field.
Add a action "Send mail" and select your added field in TO field &amp; organize your template using tokens of commerce-order.


I hope this will work, though there are other way working around code.