Not ESET related but thought id share with you what we use
Its an Exchange Rule as Marcos has said:
Apply this rule if: sender is located "Outside the Organisation"
Do the following: Prepend the disclaimer
and fall back to action Ignore if the disclaimer can't be inserted.
<table border=0 cellspacing=0 cellpadding=0 align="left" width="100%">
<tr>
<td style="background:#ff0505;padding:5pt 5pt 5pt 5pt"></td>
<td width="100%" cellpadding="7px 6px 7px 15px" style="background:#fc5858;padding:5pt 4pt 5pt 12pt;word-wrap:break-word">
<div style="color:#ffffff;">
<span style="color:#fff; font-weight:bold;">Caution:</span>
ENTER YOUR MESSAGE HERE
</div>
</td>
</tr>
</table>
<br>
<br>
<br />
Mine has colours in but you can amend as you see fit
