1. A method executed by a computer system for securing an electronic message comprised of data stored on the computer system, that is in a state of being composed as a result of a user operating a computer program running on said computer comprising: automatically detecting a change in at least one of a plurality of predetermined conditions of the data representing the electronic message during the period that the electronic message is in the state of being composed; in response to the automatic detection of the change of the at least one predetermined condition, applying at least one security policy rule to the data representing the electronic message while the electronic message remains in the state of being composed, said at least one security policy being selected by the computer system in dependence on which of the at least one of the plurality of predetermined conditions was detected as having been changed; wherein the step of applying at least one of the security policy rule comprises: determining a first security indicia for a file attachment to the electronic message; determining a second security indicia for the security privilege of the user composing the electronic message; using the first and second security indicia to determine whether to prevent the file attachment from being transmitted in the electronic message, and in dependence on such determination, either modifying a data structure associated with the electronic message to remove the file attachment from the electronic message or blocking the transmission of the electronic message.