If you'd like to prepend an external disclaimer to messages from outside the 365 organization, you can do so using the instructions below. You can also add "[EXTERNAL]: " in the subject by adding an addition action.
Logon to 365 Admin Portal
Exchange Admin Center
Create New Rule
Select "More options..." at bottom
Use the following parameters below. The HTML code is at the bottom.
HTML Code samples for disclaimer:
<div id="external-disclaimer" style="background-color: #ffeb9c; width: 100%; border: 1px solid #FFEB9C; padding: 2pt; border-style: solid; border-color: #9C6500;">CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.</div><br/>