Ari is correct. When bouncing an email, the From address is never changed, but uses the original From instead of that of the person "in the middle." Nobody who receives a bounced email should ever know from the visible headers themselves that it wasn't sent to them directly from the original sender. Replacing the original header is no different than a forward, which is not what this bug should be about.
Bug 12916 Comment 402 Edit History
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Ari is correct. When redirecting an email, the From address is never changed, but uses the original From instead of that of the person "in the middle." Nobody who receives a redirected email should ever know from the visible headers themselves that it wasn't sent to them directly from the original sender. Replacing the original header is no different than a forward, which is not what this bug should be about.