Apple can't do anything to resolve it, as it is a Yahoo problem that started about 2 weeks ago. And anyway, Apple isn't here to answer you; this is a user-to-user technical support forum.
Yahoo email has been giving a lot of people problems for a while. It's probably just incompetence or indifference, but it may also be Yahoo trying to force users to switch the the Yahoo mail app.
Yahoo problems are nothing new; I gave up Yahoo mail several years ago, due to problems back then. And their implementation of DMARC essentially prevents users from posting to any mailing list. Except Yahoo's, of course.