Overview
You would like to add the warning message automatically to the top of all emails received from outside the organization / your domain.
Solution
There is a Disclaimer possibility in MailEssentials but you cannot add disclaimers for incoming emails, only for outgoing emails.
Still, you can configure this option directly on Exchange using transport rules. Refer to these external articles for the relevant information: Appending text to emails received from external addresses or Organization-wide disclaimers, signatures, footers, or headers in Exchange Server.