Message history wouldn't use that; it uses headers (metadata) like 'In-Reply-To:' and 'References:', which are in… well, I was going to say RFC 822, but it turn

Message history wouldn't use that; it uses headers (metadata) like 'In-Reply-To:' and 'References:', which are in… well, I was going to say RFC 822, but it turns out they go back to RFC 724 from 1977. A MIME digest would be used for the case that you want to provide someone a bunch of messages they don't already have. In that case, this brings up another reason this is superior to including the thing as text: it preserves all the threading and other metadata.