如何修复 ASP.NET 中的“System.IO.IOException：无法从传输连接读取数据：连接已关闭”？

Question

我正在尝试从 ASP.NET Core 3.1 应用程序发送邮件，但我总是收到此错误消息。我该如何解决这个问题？

[我找到了一个与此类似的answer，但答案是 9 岁，对我没有帮助。]

// Exception e;
// e.Message + e.InnerException + e.GetBaseException() =
Failure sending mail. 
System.IO.IOException: Unable to read data from the transport connection: 
The connection was closed. at System.Net.Mail.SmtpReplyReaderFactory.ProcessRead(Byte[] buffer, 
Int32 offset, Int32 read, Boolean readLine) at System.Net.Mail.SmtpReplyReaderFactory.ReadLines(SmtpReplyReader 
caller, Boolean oneLine) at System.Net.Mail.SmtpReplyReaderFactory.ReadLine(SmtpReplyReader caller) 
at System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port) 
at System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port) 
at System.Net.Mail.SmtpClient.GetConnection() at System.Net.Mail.SmtpClient.Send(MailMessage message) System.IO.IOException: Unable to read data from the transport connection: The connection was closed. 
at System.Net.Mail.SmtpReplyReaderFactory.ProcessRead(Byte[] buffer, Int32 offset, Int32 read, Boolean readLine) at System.Net.Mail.SmtpReplyReaderFactory.ReadLines(SmtpReplyReader caller, Boolean oneLine) 
at System.Net.Mail.SmtpReplyReaderFactory.ReadLine(SmtpReplyReader caller) at 
System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port) at 
System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port) at 
System.Net.Mail.SmtpClient.GetConnection() at System.Net.Mail.SmtpClient.Send(MailMessage message)

这是我的邮件发送代码 -

private void sendmail(string usermail, string message)
{
    SmtpClient smtpClient = new SmtpClient("smtp.yandex.com", 465);
    smtpClient.Credentials = new System.Net.NetworkCredential("may mail address", "password");
    smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
    smtpClient.EnableSsl = true;
    MailMessage mail = new MailMessage();
    mail.Body = message;
    mail.Subject = "My Awesome Subject";
    mail.From = new MailAddress("may mail address", "Awesome User");
    mail.To.Add(new MailAddress(usermail));
    smtpClient.Send(mail);
}

Answer 1

Yandex 有一个用于外部应用程序的特殊密码，因此请在页面https://passport.yandex.ru/profile 上创建该密码。第二点是使用了错误的端口，很奇怪，但是Yandex使用的是587而不是465。

Answer 2

您需要更改以下两件事，它应该可以工作：

1. 根据answer：尝试使用端口 587 而不是 465。端口 465 在技术上已被弃用。 SmtpClient smtpClient = new SmtpClient("smtp.yandex.com", 587);

2. 由于您使用 SSL，您需要添加：System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;。欲了解更多信息，请查看answer

System.Net.ServicePointManager.SecurityProtocol - 此属性 选择安全套接字层 (SSL) 或传输的版本 用于新连接的层安全 (TLS) 协议 仅安全超文本传输​​协议 (HTTPS) 方案；现存的 连接没有改变。