I found today this patent:
US PATENT 7487217
http://www.docstoc.com/docs/57449330/Network-Domain-Reputation-based-Spam-Filtering---Patent-7487217
but then it seems prior art existed in the form of DKIM (which was started
around 2004 http://news.domainmonster.com/dkim-email/)
Not being a patent lawyer or anything like this, and knowing the US patent
office gives anyone anything, I just wanted to point to this patent.
May be also this has already been discussed a lot here...
_______________________________________________
NOTE WELL: This list operates according to
http://mipassoc.org/dkim/ietf-list-rules.html