Sending mail via gmail from C# is giving an unexpected errors

Question

I am trying to send an e-mail from my c# WinForms application using g-mail SMTP Server. Here is my code:

        string fromEmail = txtFromEmail.Text.Trim();
        string toEmail = txtToEmail.Text.Trim();
        string[] toArray = toEmail.Split(',');

        MailMessage msg = new MailMessage();
        msg.From = new MailAddress(fromEmail);
        for (int i = 0; i <= toArray.Length - 1; i++)
        {
            msg.To.Add(toArray[i]);
        }
        msg.Subject = "Test E-mail";
        msg.Body = "This is a test";
        msg.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;
        msg.HeadersEncoding = Encoding.GetEncoding(1252);
        msg.SubjectEncoding = Encoding.GetEncoding(1252);
        msg.BodyEncoding = Encoding.GetEncoding(1252);

        AlternateView av1 = AlternateView.CreateAlternateViewFromString(msg.Body, null, System.Net.Mime.MediaTypeNames.Text.Html);
        msg.AlternateViews.Add(av1);

        smtp = new SmtpClient();
        smtp.Host = txtSMTPServer.Text.Trim();
        smtp.UseDefaultCredentials = false;

        NetworkCredential cred = new NetworkCredential();

        SecureString ss = new NetworkCredential("", txtSMTPPassword.Text.Trim()).SecurePassword;
        cred.UserName = txtSMTPUsername.Text.Trim();
        cred.SecurePassword = ss;

        smtp.Credentials = cred;
        smtp.EnableSsl = chkEnableSSL.Checked;
        smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
        smtp.Port = Convert.ToInt16(txtSMTPPort.Text.Trim());
        smtp.SendCompleted += new SendCompletedEventHandler(SendCompletedCallback);


        smtp.SendAsync(msg, msg);

I am getting the following error message:

'e.Error.InnerException.Message' threw an exception of type 'System.NullReferenceException'
Data: {System.Collections.ListDictionaryInternal}
HResult: -2147467261
HelpLink: null
InnerException: null
Message: "Object reference not set to an instance of an object."
Source: "8a9e67622e334c659c856a023d4b1631"
StackTrace: "   at <>x.<>m0(frmSettings <>4__this, Object sender, AsyncCompletedEventArgs e)"
TargetSite: {System.String <>m0(Ariba.frmSettings, System.Object, System.ComponentModel.AsyncCompletedEventArgs)}

I am able to send e-mail with a non-Gmail SMTP server. What am I missing?

I have since changed my send method to synchronous using smtp.Send(msg), but now I am getting a different error:

{"Unable to read data from the transport connection: net_io_connectionclosed."}
Data: {System.Collections.ListDictionaryInternal}
HResult: -2146232800
HelpLink: null
InnerException: null
Message: "Unable to read data from the transport connection: net_io_connectionclosed."
Source: "System"
StackTrace: "   at System.Net.Mail.SmtpReplyReaderFactory.ProcessRead(Byte[] buffer, Int32 offset, Int32 read, Boolean readLine)\r\n   at System.Net.Mail.SmtpReplyReaderFactory.ReadLines(SmtpReplyReader caller, Boolean oneLine)\r\n   at System.Net.Mail.SmtpReplyReaderFactory.ReadLine(SmtpReplyReader caller)\r\n   at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint)\r\n   at System.Net.Mail.SmtpTransport.GetConnection(ServicePoint servicePoint)\r\n   at System.Net.Mail.SmtpClient.GetConnection()\r\n   at System.Net.Mail.SmtpClient.Send(MailMessage message)"
TargetSite: {Int32 ProcessRead(Byte[], Int32, Int32, Boolean)}

I realize this will probably get tagged as duplicate again, since this was probably asked before, but nothing I have read will solve my problem.

BTW: I have set my Google settings to allow unsecure apps.

Answer 1

We see the real error is here:

net_io_connectionclosed

This means GMail reject your connection.

This is expected.
GMail no longer accepts regular authentication for SMTP connections by default!
It hasn't for a few years now.

If you want to use an GMail as a traditional SMTP server, you must do one of three things:

1. Enable 2-Factor authentiation and set up a Per-App Password

or

2. Support OAuth authentication

or

3. Enable Less Secure Apps

If you don't do one of those things, GMail will reject your SMTP connections.