SmtpClient抛出IOException

Question

似乎SmtpClient在处理时抛出IOException。 邮件成功发送，没有错误。 Exeption被代码中的第二个catch块捕获， 应该处理Smtp客户端之后。 邮件已成功发送

public static async Task<String> SendMail(MailMessage mailMessage)
    {
        String errorString = "";
        try
        {
            using (SmtpClient smtpClient = new SmtpClient(SMTPServer, SMTPPort))
            {

                smtpClient.EnableSsl = StartTLS;
                if (SMTPAuthenticationRequired)
                {
                    NetworkCredential credentials = new NetworkCredential(SMTPUsername, SMTPPassword);
                    smtpClient.Credentials = credentials;
                }
                try
                {
                    await smtpClient.SendMailAsync(mailMessage);
                }
                catch (Exception ex)
                {
                    errorString += ex.Message + ";" + ex.InnerException;
                }
            }
        }
        catch (Exception ex) //Exception were catched here...
        {

        }
        return errorString;            
    }

  

例外：

"System.IO.IOException" wurde aufgefangen.
  HResult=-2146232800
  Message=Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.
  Source=System
  StackTrace:
       at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
       at System.Net.FixedSizeReader.ReadPacket(Byte[] buffer, Int32 offset, Int32 count)
       at System.Net.Security._SslStream.StartFrameBody(Int32 readBytes, Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
       at System.Net.Security._SslStream.StartFrameHeader(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
       at System.Net.Security._SslStream.StartReading(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
       at System.Net.Security._SslStream.ProcessRead(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
       at System.Net.TlsStream.Read(Byte[] buffer, Int32 offset, Int32 size)
       at System.Net.PooledStream.Read(Byte[] buffer, Int32 offset, Int32 size)
       at System.Net.Mail.SmtpPooledStream.Dispose(Boolean disposing)
       at System.IO.Stream.Close()
       at System.IO.Stream.Dispose()
       at System.Net.ConnectionPool.Destroy(PooledStream pooledStream)
       at System.Net.ConnectionPool.ForceCleanup()
       at System.Net.ConnectionPoolManager.CleanupConnectionPool(ServicePoint servicePoint, String groupName)
       at System.Net.Mail.SmtpClient.Dispose(Boolean disposing)
       at System.Net.Mail.SmtpClient.Dispose()
       at Framework.MailTool.SimpleMailTool`2.<SendMail>d__19.MoveNext() in c:\Users\Administrator\Source\Repos\syp-project-api-server2\Framework\MailTool\SimpleMailTool.cs:line 159
  InnerException: System.Net.Sockets.SocketException
       HResult=-2147467259
       Message=An existing connection was forcibly closed by the remote host
       Source=System
       ErrorCode=10054
       NativeErrorCode=10054
       StackTrace:
            at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
            at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
       InnerException: 

我很困惑;-) 请求帮助