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.
End result:
Logon to 365 Admin Portal
Exchange Admin Center
Mail Flow
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:
[EXTERNAL]
<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/>