Office 365 : Error in outlook This message couldn’t be delivered because the sending email address was not recognized as a valid sender

In Microsoft Defender security.microsoft.com/

As a Microsoft 365 Admin, open the Microsoft 365 Defender portal.

At the left menu, and below “Email & collaboration“, click on “Review“, and finally click on “Restricted entities“.

Find and pick users you wish to unblock on the Restricted Entities page by clicking on them, then from the above ribbon click on “Unblock“.

Kon Belieu