Sorry, error correction for "william" (the property names are
of course only a joke):
The "william" property can be used by MUAs to identify the
responsible sender in a mail after their border MTA verified
the MAIL FROM address with SPF, and after their MDA inserted a
corresponding Return-Path into the header.
The "william" property allows to split the responsibilities
of SPF tests at the receiver between border MTA, MDA, and MUA
in different ways, its main purpose is to prevent "phishing"
attempts.
Maybe a MUST NOT for MUAs resp. auto-responders confronted with
a mail _without_ Return-Path should be added (the FUBAR defined
in RfC 3834).
Bye, Frank