A data transmission apparatus, method and computer program capable of delivering a large-sized data file to a recipient using e-mail without the need for special installation of a software program on the side of the recipient, in which a data to be delivered is sent to a recipient as an attachment to a first e-mail, and a tool program by which to obtain the data stored in a prescribed storage area beforehand is sent to the recipient via a second e-mail if transmission error has occurred in connection with the first e-mail.