Mailing lists as 2822-Sender (was: Responsibility vs. Validity)

Scott Kitterman wrote on the DKIM list:

> On Thursday 29 November 2007 06:32, Charles Lindsey wrote:
>> Note that a mailng list expander is supposed to insert a Sender.
 
> IIRC it's a SHOULD and not a MUST, so one really can't rely on it.

Can you (Charles + Scott, or anybody else who knows it) please
check this ?  I thought that mailing lists are NOT supposed to
insert Sender (or Resent-*) header fields unless they wish to 
participate in PRA checks (RFC 4406).

Example:  Charles + Scott as authors (2822-From) post an answer
to a mailing list (with 2822-Sender Scott), what's the mailing
list software supposed to do if it doesn't support a 4406 PRA ?

 Frank (cc rfc822, maybe it's not exactly a DKIM question)