c# - Office 365 SMTP 开始触发 net_io_connectionclosed

Question

不久前，我将我的 ASP.NET C# 项目配置为通过 Office 365 发送电子邮件，但上周它开始引发大量异常。

System.IO.IOException: Unable to read data from the transport connection: net_io_connectionclosed. 
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.CheckCommand.Send(SmtpConnection conn, String& response) 
at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception) 
at System.Net.Mail.SmtpClient.Send(MailMessage message)

如何防止这种情况发生？

  MailMessage message = new MailMessage(System.Configuration.ConfigurationManager.AppSettings["smtpfrom"], email, strOnderwerp, strBody);
            message.Priority = MailPriority.Normal;
            message.IsBodyHtml = true;
            SmtpClient client = new SmtpClient(System.Configuration.ConfigurationManager.AppSettings["smtpserver"], Convert.ToInt32((System.Configuration.ConfigurationManager.AppSettings["smtpport"])));

            client.EnableSsl = Boolean.Parse(System.Configuration.ConfigurationManager.AppSettings["smtpssl"]); ;
            client.Credentials = new System.Net.NetworkCredential(System.Configuration.ConfigurationManager.AppSettings["smtpuser"], System.Configuration.ConfigurationManager.AppSettings["smtppass"]);

            client.Send(message);
            client.Dispose();

异常似乎是在 Dispose 上抛出的。

Answer 1

在我们的例子中，我们已经在使用 smtp.office365.com 端点，但突然间我们开始在我们的一台机器上接收到 net_io_connectionclosed，而相同的代码在别人身上完美地工作。调查表明，这些机器将 smtp.office365.com 解析为不同的 IP 地址，看起来其中一台服务器出现了异常。

在尝试写支持查询时，微软似乎在耍我们:

Recently, we started rejecting a percentage of connections to smtp.office365.com that uses TLS1.0/1.1 for SMTP AUTH (complete disablement will start early 2022).

仅此而已。 Switching to TLS 1.2修复了整个问题。