Encoutering IOException：无法从传输连接读取数据：连接已关闭。尝试发送邮件消息时

问：

我在 .Net 7 的 Asp.Net 核心上有一个 Web 应用程序，我需要从它发送电子邮件

这是我需要发送电子邮件的两个代码段

public bool SendMail(string name, string email, string subject, string body)
        {
            ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
            System.Net.ServicePointManager.SecurityProtocol = System.Net.ServicePointManager.SecurityProtocol | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
            MailMessage message = new MailMessage();
            SmtpClient smtpClient = new SmtpClient();
            message.From = new MailAddress("[email protected]");
            message.To.Add("[email protected]");
            message.Subject = "Contact from customer: "+name;
            message.IsBodyHtml = true;
            message.Body = "<p>" + "Name: " + name + "</p>" + "<p>" + "Email: " + email + "</p>" + "<p>" + "subject: " + subject + "</p>" + "<p>" + "message: " + body + "</p>";
            smtpClient.Port = 465;
            smtpClient.Host = "mydomain.com";
            smtpClient.EnableSsl = true;
            smtpClient.UseDefaultCredentials = false;
            smtpClient.Credentials = new NetworkCredential("[email protected]", "password");
            smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
            smtpClient.Send(message);
            return true;
        }

和

public bool SendMail(string car, string name, string email, string phone, string service_type, string pickup_date, string pickup_hour, string pickup_minute, string pickup_AM_PM, string pickup_location, string pickup_destination)
        {
            
                ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
                MailMessage message = new MailMessage();
                SmtpClient smtpClient = new SmtpClient();
                message.From = new MailAddress("[email protected]");
                message.To.Add("[email protected]");
                message.Subject = "Request to book: " + car + " from: " + name;
                message.IsBodyHtml = true;
                message.Body = "<p>" + "Send Date: " + DateTime.Now + "</p>" + "<p>" + "Name: " + name + "</p>" + "<p>" + "Email: " + email + "</p>" + "<p>" + "phone: " + phone + "</p>" + "<p>" + "Servie required: " + service_type + "</p>" + "<p>" + "Pickup date: " + pickup_date + "</p>" + "<p>" + "Pickup time: " + pickup_hour + ":" + pickup_minute + " " + pickup_AM_PM + "</p>" + "<p>" + "Pickup location: " + pickup_location + "</p>" + "<p>" + "Destination: " + pickup_destination + "</p>";
                smtpClient.Port = 465;
                smtpClient.Host = "mydomain.com";
                smtpClient.EnableSsl = true;
                smtpClient.UseDefaultCredentials = false;
                smtpClient.Credentials = new NetworkCredential("[email protected]", "password");
                smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
                smtpClient.Send(message);
                return true;

这是我所拥有的

• 所有服务器信息和凭据都正确无误
• 该代码在我的机器和生产环境中都不起作用
• 它也不适用于任何电子邮件提供商（当我第一次构建它时，它与 Outlook 完美配合，现在它也不行）
• 当我尝试通过外部测试工具（https://www.gmass.co/smtp-test）发送测试电子邮件时，它可以使用相同的设置和凭据

我搜索了关于这个问题的几乎所有问题，但没有一个解决方案有效（很多答案都说使用端口 587，但是，主机指定使用 465，当我尝试使用 587 时，它甚至在测试站点上也不起作用

编辑：我的应用程序托管在带有plesk的服务器间，以防万一

编辑 2：我已经解决了这个问题，原来这是一个宇宙巧合，生产环境和我的机器有两个独立的问题，对于机器来说，是防火墙阻止了我的尝试，至于主机，这是一个简单的凭据问题。