I found today this patent:
US 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...
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