A data communication system, an image processing apparatus, a program and a data communication method which permit a user to command a reply multicast transmission in reply to a multicast transmission with easy operations. In executing a multicast transmission of first image data, the image processing apparatus sends a job ID together with the first image data to a computer which is one of the receivers of the multicast transmission. Concurrently, the image processing apparatus stores the job ID and the receivers of the multicast transmission in relative to each other as history information. When the computer replies to the multicast transmission, the computer sends a reply e-mail including second image data and the job ID to the image processing apparatus. Then, the image processing apparatus extracts the job ID from the reply e-mail and searches the history information for receivers of the multicast transmission identified by the job ID. Thereafter, in accordance with the searched receivers, the image processing apparatus designates receivers of a reply multicast transmission and sends the second image data to the designated receivers.