Satoshi used this email address in his first announcement of bitcoin.
I have never seen anything PGP signed by Satoshi, thus the lack of a signature does not indicate forgery.
Further to that point Satoshi meticulously employed strong operational security techniques, making it very likely that he/she/they destroyed the private key after withdrawing from the community.
The evidence isn't strong either way, however it's certainly leaning towards the email being legitimate.
Received: from mail.vistomail.com (vistomail.com [190.97.163.93]) by smtp1.linuxfoundation.org (Postfix) with ESMTP id 2175813F for <bitcoin-dev@lists.linuxfoundation.org>; Sat, 15 Aug 2015 19:00:05 +0000 (UTC)
http://article.gmane.org/gmane.comp.encryption.general/12588...
( Originally posted on reddit https://www.reddit.com/r/Bitcoin/comments/3h4goc/satoshi_nak... )