Enable wp_mail() in MailPoet. See http://cfxdesign.com/email-wordpress-posts-to-subscribers-with-mandrill-and-mailpoet/

mailpoet_wp_mail_enabler.php

<?php
/*
Plugin Name: MailPoet wp_mail() Enabler
Plugin URI: https://gist.github.com/malcalevak/5ac02aa144fa1837d55bc8aeea39aeef/
Description: Enables the ability to use wp_mail() in the MailPoet plugin. Notes: Requires MailPoet v2.6.14+, which added the mailpoet_pre_config_screen action hook. It also appears this is likely to break after the release of v3. Lastly, the MailPoet code doesn't set $header, which is passed to wp_mail; as a result, you'll need to set your 'Content-type:text/html' in your header some other way (Postman SMTP has a Custom Headers field for this purpose, alternatively, you can do it more universally via the answer here: https://wordpress.stackexchange.com/questions/27856/is-there-a-way-to-send-html-formatted-emails-with-wordpress-wp-mail-function)
Version: 1.0
License: GPL-2.0+
License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 */

function mailpoet_enable_wpmail(){
  if(class_exists('WYSIJA')){
    $model_config = WYSIJA::get('config','model');
    $model_config->save(array('allow_wpmail' => true));
  }
}
add_action('mailpoet_pre_config_screen', 'mailpoet_enable_wpmail');