Yes, in most of the cases Delta tickets are refundable. If you are cancelling your e-ticket a day after buying it, then you will receive a full refund without any cancellation fee. There are chances that you may obtain a credit voucher instead of cash for non-refundable tickets and refund amount in cash for refundable tickets at any Delta airport ticket counter or over the phone (e-tickets only) for the difference in fare. A fee will apply for non-refundable tickets. Also, this does not stand valid if you have booked your tickets from an external source.