Block Domains in Office 365

Option 1: Anti-Spam Policy

Blocking a domain here will cause those messages to go into quarantine.

Go to Anti-spam policies page

https://security.microsoft.com/antispam

Click on Anti-spam inbound policy (Default)

Scroll to the bottom and click on Edit allowed and blocked senders and domains

Click on Block domains

Click Add domains

Here you can add domains

Option 2: Mail Flow Rule

Adding domains here will cause them to hard bounce back to the sender, or be silently discarded.

Go to Mail Flow Rules page

https://admin.exchange.microsoft.com/#/transportrules

Add a rule

Select The sender from the dropdown, and Domain is from the second dropdown.

Then add the list of domains and click save.

Under Do the following, select Block the message and then select either of:

• Reject the message with the enhanced status code of: If you select this, put 5.7.1 as the error code. The sender will get a bounce.
• Delete the message without notifying anyone: The sender will not get a bounce.

Click next

On the next page, Match sender address in message header or envelope.

Click next and then finish.

You also need to enable the rule.