I've posted a draft that attempts to address an attack that's begun to appear with DKIM. Interestingly, we called it out as a possible attack in RFC6376 and even RFC4871, but now it's apparently happening and being annoying enough that people (I believe from the MAAWG community) are asking if there's a protocol solution that's possible.
https://datatracker.ietf.org/doc/draft-kucherawy-dkim-rcpts/ Comments welcome. -MSK
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