The following MTAs save envelope-sender in Return-Path:
qmail 1.03 - compliant postfix 2.0.12 - compliant Exchange 2000 SP3 - compliant Exchange 5.5 SP4 - compliant sendmail 8.12.x - compliant exim 3.36 - compliant exim 4.30 -compliant Courier 0.42.2 - compliant
(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.