The email is mine. I sent it to Mike Larabel and I did not expect him to use the screenshot. I also told him I'd provide further evidence that the email was genuine; nope. The next morning I find this article.
I already addressed some concerns over the email's validity over at reddit in this thread, my username is thejoe:
http://www.reddit.com/r/linux/comments/ujwx3/valves_gabe_says_yes_to_steam_linux_this_year/
Additionally, someone posted an exchange he had with Gabe here:
http://www.gamingonlinux.com/index.php?threads/steam-really-is-coming-for-linux.890/
Short of letting you log in to my gmail account, I don't know what else I can supply. You could email Gabe yourself and ask him if you're really tenacious. I didn't intend for this explosion of interest, I didn't want the screenshot to be so widely circulated. I actually think I've annoyed Gabe by sharing emails like this.
I'm also never sharing anything with Phoronix again.