email address "spoofed"

["Tim Laureska" <hometeam () goeaston net>]

I'm not sure if "spoofing" is the correct term but here is what has
happened:

Someone is sending out email messages (with virus laden attachments)
using our domain name  ... they are addressed as coming from
support () ourdomainname com (ourdomainame would be our actual domain name)
which is an email address that we never set up.

My question is how is this done and is there anything we can do to
prevent this in the future?  The message header looks like this:

Received: from your-xhtr8hvc4p [68.55.1.207] by ourdomainname.com
  (SMTPD32-8.05) id A3C8DF0118; Fri, 05 Mar 2004 04:09:28 -0500
Date: Fri, 05 Mar 2004 04:07:07 -0500
To: info () ourdomainname com
Subject: E-mail account security warning.
From: support () ourdomainname com
Message-ID: <oruybbfoofisdsegtve () ourdomainname com>
MIME-Version: 1.0
Content-Type: multipart/mixed;
        boundary="--------hggfurquvkrsejwodibt"
X-RCPT-TO: <info () ourdomainname com>
Status: U
X-UIDL: 355598192


Tim




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