How to blacklist the domain under Office365 panel

Step 1: Login to the Office365 admin control panel.
Step 2: Go to Admin > Exchange > Mailflow
Step 3: Click the + and select Bypass Spam Filtering.
Step 4: Apply this rule if… > The sender… > domain is and enter the domain name that you wish to blacklist.
Step 5: Add the domain that you’d like blocked, and hit the plus sign to add it to your list.
Step 6: Your next step is to specify the Spam Confidence Level (SCL): click on Bypass spam filtering.
Step 7: Set SCL value it as high as you want it to be (in this case 9).
Step 8: Set the rule to Enforced.
Step 9: Click ‘Save” to apply the changes.