Info:
External Message Warning O365 Mail Rule
Apple this rule if:
The sender is located: Outside the organization
and
The recipient is located: Inside the organization
Do the following...
Prepend the disclaimer...
<p><div style="background-color:#FFEB9C; width:100%; border-style: solid; border-color:#9C6500; border-width:1pt; padding:2pt; font-size:10pt; line-height:12pt; font-family:'Calibri'; color:Black; text-align: left;"><span style="color:#9C6500"; font-weight:bold;>CAUTION:</span> 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></p>
Fallback: Wrap
