Al has some really good advice on how to fix discussion lists to work with the new Yahoo policy.
One thing I would add is the suggestion to actually check dmarc records before assuming policy. This will not only mean you’re not having to rewrite things that don’t need to be rewritten, but it will also mean you won’t be caught flat footed if (when?) other free mail providers start publishing p=reject.

dmarc • mailing lists • Yahoo