A mail server includes: first storage means that stores a model identifier and a data size corresponding to the model; receiving means that receives an electronic mail and original image data; first converting means that converts the original image data into intermediate image data; second storage means that stores the electronic mail, and the corresponding original image data or the intermediateimage data; mail transmitting means; first determining means that determines a target model on the basis of a model identifier transmitted from the destination mobile communication terminal; second determining means that determines a target data size corresponding to the target model; second converting means that converts the original image data or the intermediate image data into final image data when the target data size is smaller than the data size of the original image data or the intermediate image data.