Using classic Outlook for Windows in business environments
This has been happening since last Thursday. It's an issue with Yahoo receiving email from Microsoft. It's not something that you can do on your side. Anyone sending from Microsoft 365 to Yahoo needs to implement DKIM on their domain and Microsoft account. This requires that they turn on DKIM on their Microsoft account and add DKIM records to their domain's DNS.
You can try talking to Yahoo but they'll just blame Microsoft. Convincing your contractors that it's something they have to do might be difficult but it's currently the only way it will work - that is, until Microsoft and Yahoo work out their differences.