使用Mandrill(php)发送电子邮件时发生错误

<?php
    try {
        $mandrill = new Mandrill('YOUR_API_KEY');
        $message = array(
            'html' => '<p>Example HTML content</p>','text' => 'Example text content','subject' => 'example subject','from_email' => 'message.from_email@example.com','from_name' => 'Example Name','to' => array(
                array(
                    'email' => 'recipient.email@example.com','name' => 'Recipient Name'
                )
            ),'headers' => array('Reply-To' => 'message.reply@example.com'),'important' => false,'track_opens' => null,'track_clicks' => null,'auto_text' => null,'auto_html' => null,'inline_css' => null,'url_strip_qs' => null,'preserve_recipients' => null,'view_content_link' => null,'bcc_address' => 'message.bcc_address@example.com','tracking_domain' => null,'signing_domain' => null,'return_path_domain' => null,'merge' => true,'global_merge_vars' => array(
                array(
                    'name' => 'merge1','content' => 'merge1 content'
                )
            ),'merge_vars' => array(
                array(
                    'rcpt' => 'recipient.email@example.com','vars' => array(
                        array(
                            'name' => 'merge2','content' => 'merge2 content'
                        )
                    )
                )
            ),'tags' => array('password-resets'),'subaccount' => 'customer-123','google_analytics_domains' => array('example.com'),'google_analytics_campaign' => 'message.from_email@example.com','metadata' => array('website' => 'www.example.com'),'recipient_metadata' => array(
                array(
                    'rcpt' => 'recipient.email@example.com','values' => array('user_id' => 123456)
                )
            ),'attachments' => array(
                array(
                    'type' => 'text/plain','name' => 'myfile.txt','content' => 'ZXhhbXBsZSBmaWxl'
                )
            ),'images' => array(
                array(
                    'type' => 'image/png','name' => 'IMAGECID','content' => 'ZXhhbXBsZSBmaWxl'
                )
            )
        );
        $async = false;
        $ip_pool = 'Main Pool';
        $send_at = 'example send_at';
        $result = $mandrill->messages->send($message,$async,$ip_pool,$send_at);
        print_r($result);

    } catch(Mandrill_Error $e) {
        echo 'A mandrill error occurred: ' . get_class($e) . ' - ' . $e->getMessage();

        throw $e;
    }
    ?>

通过使用这个代码,我得到错误：

发生mandrill错误：Mandrill_HttpError – 对消息/发送的API调用失败：错误设置证书验证位置：CAfile：/usr/local/share/certs/ca-root-nss.crt CApath：none

为什么我得到这个错误？