Method of modifying a message, store-and-forward network system and data messaging system

A method of modifying a message (500) sent via a communications network data (6, 10, 14), including the message (500) transmission data (504) identifying one or more destinations of said message ( 500) and a message body (510) identifying content thereof which is specified by a sending party, the method including: identifying (801) a recipient (508) of said message (500) from the transmission data ( 504); select (805) data (916) in response to receipt of such a message (500); attaching (206) the selected data to the message content so that the message body is modified; and transmitting (207) the modified message to said one or more destinations according to the transmission data, wherein the data (916) are selected from the addressee identified in which the said selected data are not directly related to the message content.