You can add -----Reply Above This Line----- on top of email content in Front End PM PRO > Settings > Emails. Then add following code in your child theme’s functions.php

add_filter( 'fep_filter_message_before_send', function( $message ) {
	$message['message_content'] = strstr( $message['message_content'], '-----Reply Above This Line-----', true ) ?: $message['message_content'];
	return $message;
});