Let Core send emails instead of doing through MMPU.

disable-MMPU.php

<?php
/**
 * Let Core send emails instead of doing through MMPU. 
 *
 * link: TBD
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

/**
 * Disable MMPU emails. 
 */
function pmpro_disable_mmpu_emails() {
	remove_action( 'pmpro_after_all_checkouts', 'pmprommpu_send_checkout_emails' );
	remove_filter( 'pmpro_send_checkout_emails', 'pmprommpu_stop_default_checkout_emails' );
}

add_action( 'init', 'pmpro_disable_mmpu_emails' );