Is there a common X-Header for RFC 2821 "MAIL FROM"? Should it be DKIM signed?

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http://serverfault.com – w.r.t. the difference between RFC2821 MAIL From and RFC2822 FROM I'm considering having my MTA add a header specifying what was sent in the MAIL FROM portion of the envelope. The RFC2821 header is used for receiving email bouncebacks, and is the header that is checked in SPF and some SenderID configurations. The goal is to make it easier for diagnostics and debugging by having this low level information in the email header. What is an acceptable name for this SMTP header? Should this header be signed by DKIM? Is there any reason why it shouldn't be signed? (HowTos)