Forums email system security vulnerability

[sydude]

Hi guys. I'm running my forum on 3.8.4, and have been hit twice in the past week with a spammer evidently sending a ton of spam through our forum's contact us system. Can someone please advise if this is a known vulnerability that requires vB software upgrade to resolve, or if not, what other course of action I should be taking?

Here is a copy of the header of the email that was sent out:

From www.skippyforums.com@xenon2.zerolag.com Sat Jun 02 04:20:07 2012
Received: from fantasiescometrue.com ([67.201.61.149]:58927 helo=xenon2.zerolag.com)
by gator351.hostgator.com with esmtp (Exim 4.77)
(envelope-from <www.skippyforums.com@xenon2.zerolag.com>)
id 1SakVD-0000qa-0G
for steve@qwik-cash.com; Sat, 02 Jun 2012 04:20:07 -0500
Received: by xenon2.zerolag.com (Postfix, from userid 1193)
id 1FAAB8EC4E6; Sat, 2 Jun 2012 02:20:01 -0700 (PDT)
To: steve.hachey@yahoo.ca, steve.jagodic@yahoo.com, steve.jl@ntlworld.com, Steve.M.Weber@gmail.com, steve@aquamiracle.com, steve@hesketh.us, steve@leonard3.com, steve@merritt.co.uk, steve@qwik-cash.com, steve_ra1@yahoo.com, steve3843@gmail.com, steve-bmw@tesco.net, steveccowley@gmail.com, ETC ETC...

and here's the rest of the header at the end of the recipient list:

Subject: Need Work? This Company Needs Your Help!
From: "Team Juicy Racing's Skip Barber Racing School and Race Series Forums" <support@skippyforums.com>
Auto-Submitted: auto-generated
Message-ID: <20120602091959.256d07123a72@www.skippyforums.co m>
MIME-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
X-Priority: 3
X-Mailer: vBulletin Mail via PHP
Date: Sat, 2 Jun 2012 02:20:01 -0700 (PDT)
Content-Transfer-Encoding: quoted-printable
X-Spam-Status: No, score=2.4
X-Spam-Score: 24
X-Spam-Bar: ++
X-Spam-Flag: NO