Exactly why we need a standard framework for the use of stamps.
Indeed, AOL, gmail and MSN could set-up such a system for use just between themselves and benefit greatly from it. But with a standard "stamp" framework, you could choose to put up your own "stamp" generator.
So how does a sender know you require a stamp? Where do they obtain one? Once they've gotten one, how do they affix it to your e-mail (so that you can filter on it)? Those are the questions a standard should answer.
Personally, I believe that there should be a standard method for deriving a URL from e-mail addresses. That stamp generators should be placed at the derived URL, and that affixing a stamp (retrieved from the URL) should be in an "X-header". That's clearly not the only possible approach, but the beginning of the discussion I'd like to see.
Gerald