In creating email notifications for tickets via Custom Ticket Rules, I found that it's not enough just to put -+-+- Please reply above this line to add a comment -+-+- in the email in order for everything below that to be cropped out of the comment.

When I looked at the source of an email generated by Email on Events, I found this tag:  <div id="k_html_email_delimiter">

Putting that at the top of the email while in Code View made it work, even without having the "Please reply above this line" line in the email.