Your message wasn't delivered because the recipient's email provider rejected it

Heather_3578not sure if this is the fix but I had the same error on a redirect rule on an inbox. I had set up the rule a year ago and it was working fine until very recently. I followed the instructions on this guide:

https://docs.helpscout.com/article/1424-enable-external-forwarding-in-microsoft-365

The UI is slightlty different as the screenshots in the guide, but you can access the policies screen at the following link:

https://protection.office.com/antispam

I haven't tested to see if this is the fix but it's worth a try.