add_action( 'phpmailer_init', 'wpas_custom_email_notification_delimiter', 99 );
/**
 * Add the e-mail notification delimiter at the very beginning of e-mails
 *
 * @param object $phpmailer
 * @return void
 */
function wpas_custom_email_notification_delimiter( $phpmailer ) {

    if ( isset( $phpmailer->Subject ) && false !== strpos( $phpmailer->Subject, 'YOUR_FIXED_STRING' ) ) {
        $phpmailer->Body = str_replace( '<!-- AS_HEADER -->', '<div style="color:#b5b5b5">##- Please type your reply above this line -##</div>', $phpmailer->Body );
    }

}

add_action( 'plugins_loaded', 'wpas_remove_default_email_delimiter', 25 );
/**
 * Remove the default delimiter
 *
 * @return void
 */
function wpas_remove_default_email_delimiter() {
    remove_filter( 'wpas_email_notifications_body', array( WPAS_Email::get_instance(), 'add_delimiter' ), 10 );
}