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I am trying to connect to mail servers (using a TcpClient) in order to test if an email address is valid without sending an actual email (this is for a client). I am currently using GMail for testing and I am able to successfully connect to the SMTP server. However, when I try to perform a RCPT TO command it says authentication failed (530 error). I have also testing this on the live.com SMTP server and also get an authentication error.

How can I test if an email address is valid on a SMTP, using a TcpClient, without actually sending an email or needing an account on that server?

Thank you in advance for any help.

What I have tried:

Below is the code I am currently using. It hasn't been optimized yet. This will be done once I get the RCPT TO command to work successfully.

C#
using (var client = new TcpClient())
            {
                host = "smtp.gmail.com";
                var port = 465;
                client.Connect(host, port);
                // as gmail requires ssl we should use sslstream
                // if your smtp server doesn't support ssl you can
                // work directly with the underlying stream
                using (var stream = client.GetStream())
                using (var sslstream = new SslStream(stream))
                {
                    sslstream.AuthenticateAsClient(host);
                    using (var writer = new StreamWriter(sslstream))
                    using (var reader = new StreamReader(sslstream))
                    {
                        string read = "";
                        if (stream.DataAvailable)
                        {
                            read = reader.ReadLine();
                        }

                        if (!read.StartsWith("220"))
                        {
                            return false;
                        }
                        writer.WriteLine("EHLO " + host);
                        writer.Flush();

                        do
                        {
                            read = reader.ReadLine();
                        } while (read.StartsWith("250-"));


                        if (read.StartsWith("220") || read.StartsWith("250"))
                        {
                            writer.WriteLine("mail from:<test@example.com>");
                            writer.Flush();
                            read = reader.ReadLine();
                            if (read.StartsWith("530"))
                            {
                                do
                                {
                                    read = reader.ReadLine();
                                } while (read.StartsWith("530-"));
                            }
                            if (read.StartsWith("250"))
                            {
                                writer.WriteLine("rcpt to:<" + _emailAddress + ">");
                                writer.Flush();
                                read = reader.ReadLine();
                                if (read.StartsWith("250"))
                                {
                                    writer.WriteLine("quit");
                                    writer.Flush();
                                    read = reader.ReadLine();
                                    if (read.StartsWith("221"))
                                    {
                                        return true;
                                    }
                                }
                                else
                                {
                                    writer.WriteLine("quit");
                                    writer.Flush();
                                }
                            }
                            else
                            {
                                writer.WriteLine("quit");
                                writer.Flush();
                            }
                        }
                        else
                        {
                            writer.WriteLine("quit");
                            writer.Flush();
                        }
                        // gmail responds with: 220 mx.google.com esmtp
                    }
                }
            }
            return true;
Posted
Comments
MadMyche 16-Nov-18 17:28pm    
Are you really using "test@example.com" as the MailFrom value?
Dominick Marciano 16-Nov-18 18:46pm    
I originally tried that for testing purposes but I also tried using my own GMail address. Should the MAIL FROM field matter as I really only need to validate the RCPT TO value? And just for clarification, my client basically just wants it to function like this
[^]. This RCPT TO checking is the only thing I'm stuck on at the moment.

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