Could not instantiate mail function.

$config = C('THINK_EMAIL');
vendor('PHPMailer.class#phpmailer');
$mail=new \PHPMailer();
// 设置PHPMailer使用SMTP服务器发送Email
$mail->IsSMTP();
$mail->SMTPSecure = 'tls';
$mail->CharSet = 'UTF-8'; //设定邮件编码，默认ISO-8859-1，如果发中文此项必须设置，否则乱码
$mail->IsSMTP(); // 设定使用SMTP服务
$mail->SMTPDebug = 0; // 关闭SMTP调试功能
// 1 = errors and messages
// 2 = messages only
$mail->SMTPAuth = true; // 启用 SMTP 验证功能
$mail->SMTPSecure = 'ssl'; // 使用安全协议
$mail->Host = $config['SMTP_HOST']; // SMTP 服务器
$mail->Port = $config['SMTP_PORT']; // SMTP服务器的端口号
$mail->Username = $config['SMTP_USER']; // SMTP服务器用户名
$mail->Password = $config['SMTP_PASS']; // SMTP服务器密码
$mail->SetFrom($config['FROM_EMAIL'], $config['FROM_NAME']);
$replyEmail = $config['REPLY_EMAIL']?$config['REPLY_EMAIL']:$config['FROM_EMAIL'];
$replyName = $config['REPLY_NAME']?$config['REPLY_NAME']:$config['FROM_NAME'];
$mail->AddReplyTo($replyEmail, $replyName);
$mail->Subject = $subject;
$mail->MsgHTML($body);
$mail->AddAddress('xxxxxxxxx@qq.com','admin');
return $mail->Send() ? true : $mail->ErrorInfo;