A lot of proxys go up and down a lot because peoples backdoored machines go up and down a lot (as they turn their computers on and off)
So while some proxys on your list might be "dead" during one part of the day, they will be active again during other parts of the day.
If you scan a lot of proxies you should know this.
And like SpaceAce sais... if your trying to provide assistance for people in the industry to protect themselves against crooks & hitbotters, then it's in the best interest of everyone to keep a list of all the proxies -- dead and 'alive'.
