At this time, there is not a way to make autoresponders go to the billing email address.  If a donor enters one address for the donor email on a donation form and enters another address for the billing email, the autoresponder will go to the donor email.  The donor email address is the field that is used to match to an existing constituent record or to create a new record.

If a donor would like to receive a receipt to a different email address, you can update the email address on the constituent profile and regenerate the receipt (BB746066). Please note that the receipt will still reflect the billing information as entered at the time of the transaction.