spam filter - Exchange 2013 - Remove warning text from outgoing email body

I created a transport-rule in our Exchange server 2013 where it will add a warning text on top of email-body to all external incoming emails. This is to alert employees about potential risks in external emails when it has website-links and attachments which may be harmful. The text is as follows:

Text

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.

Now, when user will reply to the email, I want it to be remove when Exchange process it to send. How can I remove the warning text from outgoing emails in Exchange? I was looking for something in rules, but there is none I could find.

Any help will be appreciated. Thanks.