Image data communication equivalent to facsimile communication using a public network is realized on e-mail communication. When a communication apparatus for transmitting/receiving e-mail data by connecting to the Internet is to communicate e-mail data having facsimile-format image data attached, communication concerning functional information is performed in addition to the communication of the e-mail data. The functional information can be exchanged by an optimum method matching the form of connection to the Internet. This allows maximum utilization of the capability of each apparatus function in facsimile image communication using e-mail.