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This would allow filters like SpamAssassin to pick up the envelope-from used at each step of the chain, which is very valuable especially when intermediate mailing lists steps tend to rewrite it. Also note that (For example, fetchmail makes some incorrect assumptions, and will add an *incorrect* Return-Path header if an X-Envelope-From header exists, even from an earlier handover; this avoids that issue.)

If I recall correctly, some versions of exim seem to support this.

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