Override FreeSSL WordPress plugin domain exclusion list

functions.php

<?php
// Override FreeSSL plugin domain exclusion list
add_filter('option_exclude_domains_auto_install_free_ssl', function ($data) {
    $custom_data = array('domains_to_exclude' => array('mail.mydomain.com'));
    if(isset($data['domains_to_exclude'])){
        $data['domains_to_exclude'] = array_merge($data['domains_to_exclude'], $custom_data['domains_to_exclude']);
        return $data;
    }
    return $custom_data;
});