RFC2822 says:
Resent fields are used to identify a message as having been
reintroduced into the transport system by a user. The purpose of
using resent fields is to have the message appear to the final
recipient as if it were sent directly by the original sender, with
all of the original fields remaining the same. Each set of resent
fields correspond to a particular resending event. That is, if a
message is resent multiple times, each set of resent fields gives
identifying information for each individual time. Resent fields are
strictly informational. They MUST NOT be used in the normal
processing of replies or other such automatic actions on messages.
Note: Reintroducing a message into the transport system and using
resent fields is a different operation from "forwarding".
"Forwarding" has two meanings: One sense of forwarding is that a mail
reading program can be told by a user to forward a copy of a message
to another person, making the forwarded message the body of the new
message. A forwarded message in this sense does not appear to have
come from the original sender, but is an entirely new message from
the forwarder of the message. On the other hand, forwarding is also
used to mean when a mail transport program gets a message and
forwards it on to a different destination for final delivery. Resent
header fields are not intended for use with either type of
forwarding.
The resent originator fields indicate the mailbox of the person(s) or
system(s) that resent the message. As with the regular originator
fields, there are two forms: a simple "Resent-From:" form which
contains the mailbox of the individual doing the resending, and the
more complex form, when one individual (identified in the
"Resent-Sender:" field) resends a message on behalf of one or more
others (identified in the "Resent-From:" field).
Note: When replying to a resent message, replies behave just as they
would with any other message, using the original "From:",
Question 1: Does this mean that maling list expanders should,
or should not, add resent-headers? Or does the text allow
either alternative?
Question 2: In case the text allows either alternative, which
is the proper way for a mailing list handled to do? Should it
add resent-headers or not?