Someone found a new vulnerability in SendMail this week. SendMail is a Unix based SMTP mail server. SendMail's roots are REALLY OLD in internet time. Robert Tappan Morris exploited a flaw in SendMail back in 1988 that took down a good part of the internet that existed at that time.
ALOT of people have look at SendMail's code over the years and there are still vulnerabilities. Yes, I know the code has been upgraded, but goodness, this one really old Unix app that still has security problems. If we still see issues in old code like this, it's no wonder we still see issues with IE6.