Envelope-Sender data in headers
The following MTAs save envelope-sender in Return-Path:
(list from spf-discuss list, thread 'Re: [spf-discuss] Is Return-Path as available as we think?', Date: Wed, 28 Jan 2004 06:31:05 -0600 (04:31 PST).)
Also see EnvelopeSenderInReceived for a proposal to come up with a better way to store envelope-sender data in the headers, resilient in the face of multiple relays.