ANswer: Gabe emailed everyone that wanted to talk to him.http://www.halflife2.net/forums/showthread.php?s=&threadid=1298&perpage=15&pagenumber=1)
Now the general user, you & me, doesn't care about the importance of mail headers & trailers. But the hacker sees this as doorways. So they set up a backtrail & find the ISP, log the networking on the ISP (don't ask me how), & find the timeline they are looking for, and see the IP. Once they have that, they can gain access through the net.
The problem Valve's Gabe Newell made in my opinion, this isn't for sure fact, was that although he is a really really cool guy & people like him from a gamer stand point & he talks to the gamers, was that he was emailing from a non-secure system on the same LAN as the rest of Valve's computers. This is just plain stupid. It is asking for trouble. Let's say that we have this really smart carmack-like coder that is just itching for Halflife2 & he hates steam & he is really completely pissed off that HL2 is going to be postponed. He could be a fan. He could ask a very obvious question just to get a response & then trace it back & infiltrate the system.
The point I'm trying to make is that I think that Gabe, although very cool to talk to his fans, was a dipshit moron & had no idea and no clue that idiots are on the net & will fuck you given the chance. I think that some fuckwad was pissed & learned how to piss up the rope of Gabe's email account. Pretty foolish for Gabe to be talking on the same connection that the source code was on in the first place. Seperate that shit! Talk to people, man that is soo cool, but be smart!