For context, here’s a description of the issue from the IETF discussion list:
In short, the problem is that Yahoo’s mail servers now have a DMARC policy that declares that any message with a “From” header at yahoo.com which wasn’t sent by Yahoo’s servers is invalid and should be discarded. Since mailing lists normally forward messages on while keeping the original sender, this makes anything they forward from a Yahoo user look like junk!
The workaround you’re thinking of that’s available in some newer Mailman versions is to make all messages from the list always show up with the list address as a sender. This is a poor solution, though, as it makes it much more difficult to search through your mail for messages sent to the list by a particular person… ultimately, Yahoo is probably going to need to change their DMARC policy, as the one they’ve got in place right now is simply too strict.