Stationery components do not apply to email replies or forwards, or they are applied to the bottom of the email only.
The HTML and Plain Text construction of the Stationery Layout will need to be modified with appropriate tags.
Add one of the following Mimecast HTML tags:
<MC Type='Comment'>inserts that last reply from the email chain only.
<MC Type='Previouscomment'>inserts the rest of the email chain, excluding the latest reply.