As I understand it, they were originally using Solaris, not Linux. Hotmail is also using currently using qmail. Actually it is both FreeBSD and Solaris. It was all FreeBSD at one point but threads stuff was a little weak when the people over at HotMail needed them. So they had to go to Solaris for part of their operation.
More research appears indicated.
One of the big mail systems was doing that. I would exclude those that have to run as root. Or the recent mp3 decoder bug. Gosh, now I'm getting paranoid. Hopefully this has been fixed, but it's taking a ton of time for them to do it.