Why can't I send email from Outlook (microsoft account) to a Yahoo or AT&T email account?

For years, I have been sending emails from Outlook on my desktop computer and mobile device with no problems. But the last few days, I am getting rejection messages saying my emails cannot make it through. Can I get help figuring out what is going on?

Here is the undeliverable error message I got when trying to send to an att.com email address (came from here: Mail Delivery Subsystem <******@xxxxx.prodigy.net>):

"The original message was received at Tue, 16 Jan 2024 12:28:35 -0800 from mail-mw2nam12on2049.outbound.protection.outlook.com [40.107.244.49] ----- The following addresses had permanent fatal errors ----- (reason: 554 [RL02] Message content not accepted for policy reasons; see https://postmaster.yahooinc.com/error-codes) ----- Transcript of session follows ----- ... while talking to mx-sbc.mail.am0.yahoodns.net.: >>> DATA <<< 554 [RL02] Message content not accepted for policy reasons; see https://postmaster.yahooinc.com/error-codes 554 5.0.0 Service unavailable"

And here is the undeliverable error message I got multiple times when trying to send to a yahoo.com email address (came from: Microsoft Outlook <******@xxxxxxx.com>):

"Delivery has failed to these recipients or groups:

(******@yahoo.com)
Your message couldn't be delivered. Despite repeated attempts to contact the recipient's email system it didn't respond.

Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that their email system isn't accepting connection requests from your email system. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem.

For more information and tips to fix this issue see this article: https://go.microsoft.com/fwlink/?LinkId=389361.

Thank you for help.