A new sign that should be posted and enforced around the world. Of course, we will have some common sense standard of security which each e-mail client will be tested against, and those that fail will find themselves like the kids who are too short for the roller coaster at the amusement park - turned away, forced to wait until things have changed.
This should make the internet a little bit of a better place, as you won't get crappy clients like Lookout causing the net to sputter and find itself clogged from those horrible programs that attack the Lookout virus distribution trojan.