Delivery has failed to these recipients or groups [...]
Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send email. Contact your email admin for assistance.
This means their account may have been compromised and used as phishing bait.
To fix this as a Microsoft 365 administrator, navigate to https://protection.office.com/restrictedusers and unblock the user's account.
The best practice to prevent hacking is to activate multi-factor authentication (MFA) for all users at the following link: https://account.activedirectory.windowsazure.com/UserManagement/MultifactorVerification.aspx
