Maybe you did not understand my message. I know what you say. However:
I see a message from a list as a message-from-a-list , not as a forwarded-message-from-a-list-user. Because: How can a user authorize someone to send a message on behalf of his/her name (by sending an email). This should not ever happen. Example: A bank sends me an email which was authorized (in some way). I now forward this message. The message is genuinely not modified. But it still does not authorize me to send this email pretending to be the bank, even if it is the same message. Conclusion: If an email was sent by me, it should be authorized/authenticated by me.
For mailing lists you might want to indicate that the message can be interpreted as being forwarded for a specific user. In that way the user-interface of the email client can reply to a user directly instead of the mailing list. If that is what one wants.
MailPlus B.V. Netherlands (ESP)