Phishing Protection through Exchange Online Mail-Flow Rule

It is possible to create mail-flow rules that prevent phishing attempts impersonating internal Microsoft 365 users. The mail-flow settings are mentioned below,

Name: Admin defined

Apply this rule if: Sender is located Outside the Organization And The sender domain is <Organization's email domain>

Action:

Prepend the Subject of the Message with: <Admin selected message such as Warning: Possibly Phishing Email>

and 

Set the spam confidence level (SCL) to: 8

Note: Admin can further customize the rule as per their requirement and must whitelist IP addresses in case they are using 3rd party services for emailing. An example rule screen is mentioned below,