Unless you know the capabilities of your own mail user agent AS WELL AS
those of your subscribers, this might be an exercise in futility.

Subscribers must have their distribution options set to be something
other than SHORT e.g., FULLHDR seems to work for those mail clients.

Also, presumably, your list does not prohibit HTML text from being
distributed (1.8d beta).
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Those headers aren't "forged" just creatively massaged