Setting envelope sender in postfix

I am shared hosting customer and have some forwarding (alias) addresses for which I would like Postfix to rewrite the envelope sender to a list owner address, as it does for mailman lists. A quick web search suggests that Postfix will automatically do this if I set up owner-$address for each of my forwarding addresses. I’ve tried this but the sender is still the original sender. This makes some recipients sad if the sender has set SPF.

Thoughts? Is the required address spelled differently for this postfix installation? Is there some setting I need to apply? Is this feature only available if I have a VPS or something?