All mailing list forward mail with the original sender in the enveloppe
Not true: Mails from the swinog-mailinglist reach my mailserver with swinog-bounces@lists.swinog.ch as sender envelope address
and if you forward your mail on one server to another one with e.g. a simple .forward rule it will also re-use the same envelope from address. Forwarding mail is very common and it is important to use the same envelope form address in the forwarding path. Everybody who denies this fact does not understand the email system and the way bounces work.
If you chose to forward mail this way, then you'd better make sure that the destination mail server doesn't apply spf checks for mails coming from the relaying server. or that the forwarding server rewrites the sender address.
If you're a provider that allows its users to forward their mail to remote addresses, then I'd advise you to use sender rewriting and thus offer your customers a reliable mail relaying. That's a marketing argument.
But in the end, everyone can do whatever they want to do ;-)
Jean-Pierre