mailpoet_smtp_secure_connection_options_hook.php

<?php

add_filter('mailpoet_mailer_smtp_options', 'mailpoet_secure_connection_options');

function mailpoet_secure_connection_options() {
  return [
      'ssl' => [
        // description of available options can be found here: http://php.net/manual/en/context.ssl.php
        'verify_peer' => false, 
        'verify_peer_name' => false,
        'allow_self_signed' => true,
      ],
  ];
}