An electronic message modifying system, comprising: [a] a sender moiety comprising at least one constituent selected from the group consisting of: a sending terminal for composition/formatting of an electronic message and a message generator for generating numerous electronic messages; [b] a recipient moiety comprising a message user agent, said message user agent being configured to receive messages sent from said sender moiety and to present said messages for view of a recipient; [c] a communication network between said sender moiety and said recipient moiety; [d] at least one modification parameters database, said database containing a plurality of modification parameters; [e] at least one message modification agent for receiving incoming messages, said message modification agent being operable to retrieve an updated set of said modifications parameters from said database and to subject said incoming messages to a modification; wherein said modification comprises modifying at least one constituent of said message intended for said recipient, wherein said modification is performed in accordance with at least one modification parameter retrieved from said database; characterized in that said modification parameters in said database relate to at least one member selected from the group consisting of: at least one quality/characteristic of a machine associated with said recipient message user agent, at least one quality/characteristic of a communication service provided by a network operator for said machine associated with said recipient message user agent, at least one quality/characteristic of an external service provider, other than said network operator, for said machine associated with said recipient message user agent. [a] a sender moiety comprising at least one constituent selected from the group consisting of: a sending terminal for composition/formatting of an electronic message and a message generator for generating numerous electronic messages; [b] a recipient moiety comprising a message user agent, said message user agent being configured to receive messages sent from said sender moiety and to present said messages for view of a recipient; [c] a communication network between said sender moiety and said recipient moiety; [d] at least one modification parameters database, said database containing a plurality of modification parameters; [e] at least one message modification agent for receiving incoming messages, said message modification agent being operable to retrieve an updated set of said modifications parameters from said database and to subject said incoming messages to a modification; wherein said modification comprises modifying at least one constituent of said message intended for said recipient, wherein said modification is performed in accordance with at least one modification parameter retrieved from said database; at least one quality/characteristic of a machine associated with said recipient message user agent, at least one quality/characteristic of a communication service provided by a network operator for said machine associated with said recipient message user agent, at least one quality/characteristic of an external service provider, other than said network operator, for said machine associated with said recipient message user agent.