That is a comment line from the HTML. It is hard to say why it is displaying in your text email. Can you post the HTML from your email here so we can get a better look at it? Right Click On Email > Download HTML.
If this is not appreaing in the editor, in all likelihood it is included in your global footer. You should check in Admin>Email and then see if this text is included in the Text Only footer.
In the HTML version of the email, this comment is processed by the HTML renderer so the comment does not appear. The text version of the email does not process this text, so it will appear to the leads who receive that version.