Filter mailing-list messages based on sender

[wvengen]

In #2 Postfix was configured to lookup mailing-list addresses in the database.
One thing was missing here: everyone can send to all mailing-lists. To avoid spam, some filtering would be useful.
How exactly, would be good to look up in the existing system. In my memory, that would be to only allow list members to send messages to the list.

[wvengen]

In the previous config, this was configured (partly?) in the 'routers' section of exim.conf, notably senders. Not sure how this all connects together, though.