The suggestion in the comment by SebastianNielsen is exactly what is done by the from_is_list Wrap Message setting first implemented in Mailman 2.1.16 and made more generally available (not requiring site option to enable) in Mailman 2.1.18. See DEV/DMARC.
MailmanWiki: DEV/DKIM/0005 (last edited 2015-03-15 01:11:20 by msapiro)