The concerns are legitimate and need to be (are being) addressed, however,
"sender rewriting" doesn't seem to require any changes except at providers
of forwarding services, it's completely backwards compatible, as far as I
can tell, and doesn't call for any extensions. It may not be as technically
"neat" as other suggestions, but I'd guess it has a much better chance of
Sender rewriting provides a whole new envelope, while Brett's "VIA" would
provide for adding to the envelope.
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