Office 365 / Exchange Online Block Sender Domain
The procedure is the same whether you use the Classic Exchange admin center or the new Exchange admin center.
Go to Mail flow – Rules – Create a new rule
Give the new rule a name and click on More options …
Click on Apply this rule if … and select The sender … and domain is
Add the domain you want to block.
Click on Do the following … and select Block the message … – delete the message without notifiying anyone or one of the other two options regarding your needs.