PHPMailer：无法连接到 SMTP 主机。连接失败。stream_select（）

问：

我搜索了 SO 并测试了所有各种设置更改，但找不到问题的解决方案。

服务器：Windows 服务器 2019 - IIS 10

PHP格式：PHP 5.6（是的，我知道它已经过时了，但我被迫在所有服务器完全关闭之前没有时间大修一堆代码）。

PHP机器版本： 6.8.1

SMTP错误：无法连接到 SMTP 主机。连接失败。stream_select（）：无法选择 [11]：资源暂时不可用 （max_fd=656） 无法发送消息。邮件

我在Classic ASP for SMTP中尝试了相同的登录详细信息，它起作用了，所以我知道SMTP信息是正确的。

    $mail = new PHPMailer(true);
    $mail->SMTPDebug = 1;
    $mail->isSMTP();
    $mail->Host = "myHost";
    $mail->SMTPAuth = true;
    $mail->Username = "[email protected]";
    $mail->Password = "passWord";
    $mail->SMTPSecure = PHPMailer::ENCRYPTION_SMTPS; //even tried "ssl" and "tls"
    $mail->Port = "465"; //465/587
    $mail->XMailer    = ' ';

    $mail->SMTPOptions = array(
        'ssl' => array(
            'verify_peer' => false,
            'verify_peer_name' => false,
            'allow_self_signed' => true
        )
    );

错误日志：

2023-10-16 19:17:44 Connection: opening to ssl://smtp.example.com:465, timeout=300, options=array()
2023-10-16 19:17:44 Connection: opened
2023-10-16 19:17:44 Connection failed. Error #2: stream_select(): unable to select [11]: Resource temporarily unavailable (max_fd=764) [SMTP.php line 1238]
2023-10-16 19:17:44 SERVER -> CLIENT:
2023-10-16 19:17:44 Connection: closing due to error
2023-10-16 19:17:44 Connection: closed
SMTP Error: Could not connect to SMTP host. Connection failed. stream_select(): unable to select [11]: Resource temporarily unavailable (max_fd=764)
Message could not be sent.Mailer Error: SMTP Error: Could not connect to SMTP host. Connection failed. stream_select(): unable to select [11]: Resource temporarily unavailable (max_fd=764)SMTP server error: Connection failed. Detail: stream_select(): unable to select [11]: Resource temporarily unavailable (max_fd=764) SMTP code: 2