How can I intercept and stop the thunderbird send mail event?

Question

Possible Duplicate:
Stop Email From Sending

I need to intercept the send email event in Thunderbird, and prevent the sending of the email if certain conditions are met. I have managed to intercept the event with the code below, but it always sends the message after the handler, regardless of return value, or if an exception is thrown.

var composerContainer = function () {
  return {
    sendEventHandler : function (event) {
      //.....
      return false;
    }
  }
}();
window.addEventListener( "compose-send-message", composerContainer.sendEventHandler, true );

I can access and modify the contents of the message, but I have no way of stopping the sending of the message.