无法通过ASP.NET应用程序发送电子邮件

我有一个ASP.NET应用程序运行在专用服务器"Windows server 2012 R2"上。该网站始终使用SSL加密，证书正确安装在服务器上，并在通过不同浏览器访问时显示为有效。

我现在想通过我的应用程序发送电子邮件，为此，我按照本文中的说明设置了一个SMTP服务器。http://www.vsysad.com/2014/09/setup-and-configure-smtp-server-on-windows-server-2012/

按照上面指定的方式设置SMTP服务器后，我可以使用powershell发送电子邮件，并且发送电子邮件时不会出现问题。然而，当我通过我的应用程序进行同样的操作时，我会得到以下错误

Exception Type: System.Security.Authentication.AuthenticationException
Exception: The remote certificate is invalid according to the validation procedure.
Stack Trace: 
   at System.Net.Security.SslState.StartSendAuthResetSignal(ProtocolToken message, AsyncProtocolRequest asyncRequest, Exception exception)
   at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Net.TlsStream.ProcessAuthentication(LazyAsyncResult result)
   at System.Net.TlsStream.Write(Byte[] buffer, Int32 offset, Int32 size)
   at System.Net.Mail.SmtpConnection.Flush()
   at System.Net.Mail.EHelloCommand.Send(SmtpConnection conn, String domain)
   at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint)
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   at EmailSender.SendMailSingleRecipient(String recipient, String subject, String message)

从我的应用程序发送电子邮件的代码非常简单，如下所示。

MailMessage mailMessage = new MailMessage();
mailMessage.From = new MailAddress("no-reply@mysite.com");
mailMessage.To.Add("myemail@email.com");
mailMessage.Subject = "subject"
mailMessage.Body = "message"
SmtpClient smtpClient = new SmtpClient("localhost", 25);
smtpClient.Send(mailMessage);

我已经多次看到同一问题的报告，但没有解决问题的答案。有人能解释一下这个错误背后的原因以及可能的原因/解决方案吗。

protected void SendMail()
    {
        var fromAddress = "youremail@gmail.com";// Gmail Address from where you send the mail
        var toAddress = YourEmail.Text.ToString();
        const string fromPassword = "password";//Password of your gmail address
        string email = YourEmail.Text.ToString();
        string body = "First Name: " + YourName.Text + "'n";
        body += "Last Name: " + YourlName.Text + "'n";
        body += "Email: " + YourEmail.Text + "'n";
        body += "mobile no.: " + Yourmobi.Text + "'n";
        body += "Question: 'n" + Comments.Text + "'n";
        var smtp = new System.Net.Mail.SmtpClient();
        {
            smtp.Host = "smtp.gmail.com";
            smtp.Port = 587;
            smtp.EnableSsl = true;
            smtp.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
            smtp.Credentials = new NetworkCredential(fromAddress, fromPassword);
            smtp.Timeout = 20000;
        }
        smtp.Send(toAddress, fromAddress, email, body);
        try
        {
            lblMsgSend.Text = "Your Comments after sending the mail";
            lblMsgSend.Visible = true;
            Yourmobi.Text = "";
            YourEmail.Text = "";
            YourName.Text = "";
            YourlName.Text = "";
            Comments.Text = "";
        }
        catch (Exception) { }
    }

SmtpClient client = new SmtpClient("localhost", 25);
client.DeliveryMethod = SmtpDeliveryMethod.SpecifiedPickupDirectory;
client.PickupDirectoryLocation = @"C:'inetpub'mailroot'Pickup";
client.Send(mail);