US PATENT 7487217
http://www.freepatentsonline.com/7487217.html
but then it seems prior art existed in the form of DKIM (which was
started around 2004 http://news.domainmonster.com/dkim-email/)
This isn't a patent about authentication, it's about spam filtering
using the reputation of domains associated with mail. Since it's from
Microsoft, the description uses Sender ID but the claims, which are
what counts, are phrased more generally.
It was filed in 2005, so offhand I don't know how hard it would be to
find earlier discussions of reputation based filtering.
R's,
John
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