A list that allows public posting and strips unnecessary headers is the
key.  With RiceMail now sending its postings to Mailer via BSMTP, the
headers can be rather freely munged at will.  Mailer will add its Received:
from the actual originator, but this will be promptly stripped back out by
Listserv making it look (il)legitimate.
 
If the list is set for private posting then the origin will not match to an
existing subscriber, it will be rejected.  Of course, they could signup with
the bogus From: but the attempt to mail out the subscription notification
will bounce to the postmaster or listowner.
 
With that said, there are possibilities of "impersonating" another subscribed
user however.  But e-mail never has been and probably never will be anywhere
near secure.
 
/Jeff/