Instructables Newsletter wrongly rated as SPAM caused by SORBS listing Answered

The newsletter sender is listed on the SORBS anti SPAM list and therefore gets rated as spam.

An excerpt from the mail header:

The Newsletter sender is listed on the SORBS List:
X-HE-Spam-Level: +++++
X-HE-Spam-Score: 5.6
X-HE-Spam-Report: Content analysis details:   (5.6 points)
  pts rule name              description
---- ---------------------- --------------------------------------------------
-0.0 RCVD_IN_DNSWL_NONE     RBL: Sender listed at http://www.dnswl.org/, no
                             trust
                             [167.89.7.199 listed in list.dnswl.org]
  2.5 RCVD_IN_SORBS_HTTP     RBL: SORBS: sender is open HTTP proxy server
                             [167.89.7.199 listed in dnsbl.sorbs.net]
  0.8 HTML_IMAGE_RATIO_02    BODY: HTML has a low ratio of text to image area
  0.1 HTML_MESSAGE           BODY: HTML included in message
  1.5 GENERIC_IXHASH         BODY: iXhash found in ix.dnsbl.manitu.net
  0.7 MIME_HTML_ONLY         BODY: Message only has text/html MIME parts
-0.1 DKIM_VALID             Message has at least one valid DKIM or DK signature
  0.1 DKIM_SIGNED            Message has a DKIM or DK signature, not necessarily valid
X-SPAM-FLAG: Yes
X-HE-SPF: PASSED