My emails to yahoo users all of sudden kick-back as underliverable.

Dear 

Good day!

Thank you for posting to Microsoft Community. We are glad to assist!

Based on your description regarding "My emails to yahoo users all of sudden kick-back as underliverable." My sincerest apologies for the inconvenience this might be causing you. Kindly try to add a couple of CNAME records:

selector1._domainkey [for details see what admin center Microsoft 365 admin center (With Admin credentials) --> settings --> Domains]

selector2._domainkey ...

And then add a TXT record: v=spf1 include:spf.protection.outlook.com -all

And finally:

Go to Microsoft 365 admin center-->Choose Security under Admin centers--> choose Policies & Rules under Email & collaboration--> Threat Policies --> Email authentication settings-- > DKIM--> [select domain] --> enable

Kindly make sure that all your DNS records settings set correct Connect your DNS records at Wix to Microsoft 365 - Microsoft 365 admin | Microsoft Learn

For extra reference you may also refer to this thread Undeliverable: Yahoo, Gmail, and AOL domains? - Microsoft Community

If the issue persists, I therefore believe that this issue should be further investigated by our online technical support team. A support Engineer can further connect with you remotely, for further troubleshooting and involve the backend team where need be. The team might request for your public IP address or collect some logs. Please understand that as public forum support, we don’t have higher permissions to check and analyze these logs or check the IP addresses and considering your scenario maybe inconveniencing, to quickly help assist, kindly contact your organization admin or you can create a service request to report this symptom to our online channel through the Microsoft 365 Admin center or Teams admin center through the Help & Support button on the bottom right of the page. Here is contact support document: Get support - Microsoft 365 admin | Microsoft Learn

Thanks for your precious time and your understanding would be highly appreciated. Have a nice day!

Best regards,

Inema | Microsoft Community Moderator

I tried the instructions above and it only works if you have a business/corporate/school account. I am using my personal email with my personal Microsoft account and still getting Yahoo emails bounced back.

Same here, all of the sudden all emails I send to yahoo recipients get kicked back as undeliverable.