I don't believe it's that complex, and I do believe it's worth the effort in exchange for being able to tell with certainty which entity (by signature; which DNS domain) is responsible for creating each part of a message. You can then attribute parts of the text to different entities - the original author, or the mailing list signature.
And if a message is bad then it's possible to derive where the badness was introduced - something not possible with DKIM or ARC if a message has been modified. I have a draft for a method at: https://datatracker.ietf.org/doc/draft-gondwana-dkim2-modification-alegbra/ It can be used to describe all "add text" cases quite nicely, as well as wrapped structures where an existing message gets moved into a multipart/mixed with more content at the end. There's still some testing to be done for the most complex cases - but this doesn't have to be a two-way algorithm, is just has to allow describing how to convert a new email body back to the original email body, and I believe this can be done reliably and at a reasonable cost, though it could definitely use some more examples. I'm going to publish an update with another mechanism which reduces the cost of the "remove an attachment" version to at least not fill the headers with tons of junk. It doesn't reduce the message size though, because you do need to be able to recreate the old message. And I do agree there needs to be a way to say "I made changes, and I'm not telling you how to undo them" as well. Cheers, Bron. -- Bron Gondwana, CEO, Fastmail Pty Ltd br...@fastmailteam.com
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