Not sure if some of the test messages are a result of the dmarc change but you should read the article and see if it fights your symptoms, but so far not much can be done.

from the article below  "On April 8, Yahoo implemented a new DMARC policy that essentially bars any Yahoo user from accessing mailing lists hosted anywhere except on Yahoo and Google.  ...  Their best proposed solution is to ban Yahoo email users from mailing lists and encourage them to switch to other ISPs. ..."