How to enable notification to be sent to users if emails are quarantined?

Step 1: Sign into Office 365 admin portal via https://portal.office.com

Step 2: Click on Admin from the left pane and navigate to Security center.

Step 3: In the Security & Compliance center, navigate to Threat management > Policy.

Step 4: Click on the Default policy and then click Edit policy.

Step 5: Switch to Malware detection response tab.

Step 6: Select the Yes and use the default notification text option.

Step 7: Click Save.