Viruses also forge the originating mail system. Several groups maintain "blacklists" of supposed spam and virus mail servers, and other mail systems use the blacklists to block receiving mail from those systems. Several times my mail server's been added to the blacklists erroneously, and all of a sudden my incoming mail drops sharply. What really fries me is that it can take weeks to get removed from a blacklist. All in all, that approach to spam and viruses is vigilantism at its worst, because it punishes the innocent more than the guilty. We need a better way to deal with the problem.