In addition to the notice that is already displayed within the footer of all outgoing emails ("This email was auto-generated. Please do not reply."), This simple and straight forward solution will add another notice to the top, just above the salutation.
Control Panel > Languages > Search / Edit Language Strings
>> Choose Specific Language File:
mailer.phpFor the "EMAIL_SALUTE" field, REPLACE WITH
<center>**PLEASE DO NOT REPLY TO THIS EMAIL. IT IS NOT MONITORED**</center><br><br>Hello %%USERNAME%%,
or
Control Panel > Languages > Search / Edit Language Strings
>> Search String: "Hello %%USERNAME%%,"
REPLACE WITH
<center>**PLEASE DO NOT REPLY TO THIS EMAIL. IT IS NOT MONITORED**</center><br><br>Hello %%USERNAME%%,
or
MANUALLY EDIT templates/english/mailer.php
FIND
$ubbt_lang['EMAIL_SALUTE'] = "Hello %%USERNAME%%,";
REPLACE WITH
$ubbt_lang['EMAIL_SALUTE'] = "<center>**PLEASE DO NOT REPLY TO THIS EMAIL. IT IS NOT MONITORED**</center><br><br>Hello %%USERNAME%%,";
The result will be: