Yahoo has recently implemented DKIM requirements for emails. https://senders.yahooinc.com/best-practices/
In order to enable DKIM you have to access your email provider and enabled DKIM. You will then have to add a TXT record to your DNS records at your domain’s registrar.
https://support.google.com/a/answer/180504?hl=en
https://learn.microsoft.com/en-us/defender-office-365/email-authentication-dkim-configure
If TDO hosts your email and domain we are undertaking the process for you.
If you have questions or need assistance please call TDO Support or email support.tdo4endo.com