[Steven Zahm]

@ Richard

Alrighty, here the code:

add_filter( 'cn_email_message', 'cn_append_email_message' );

function cn_append_email_message( $message ) {

    $append = "\r\r" . 'This email has been sent via the ' . get_bloginfo( 'name' ) . ' website.';

    return $message . $append;
}

I did not test it, but it should work. Let me know how it goes.