SmtpClient with Gmail

Question

I'm developing a mail client for a school project. I have managed to send e-mails using the SmtpClient in C#. This works perfectly with any server but it doesn't work with Gmail. I believe it's because of Google using TLS. I have tried setting EnableSsl to true on the SmtpClient but this doesn't make a difference.

This is the code I am using to create the SmtpClient and send an e-mail.

this.client = new SmtpClient("smtp.gmail.com", 587);
this.client.EnableSsl = true;
this.client.UseDefaultCredentials = false;
this.client.Credentials = new NetworkCredential("username", "password");

try
{
    // Create instance of message
    MailMessage message = new MailMessage();

    // Add receiver
    message.To.Add("[email protected]");

    // Set sender
    // In this case the same as the username
    message.From = new MailAddress("[email protected]");

    // Set subject
    message.Subject = "Test";

    // Set body of message
    message.Body = "En test besked";

    // Send the message
    this.client.Send(message);

    // Clean up
    message = null;
}
catch (Exception e)
{
    Console.WriteLine("Could not send e-mail. Exception caught: " + e);
}

This is the error I am getting when I try to send an e-mail.

Could not send e-mail. Exception caught: System.Net.Mail.SmtpException: Message could not be sent. ---> System.IO.IOException: The authentication or decryption has failed. ---> System.InvalidOperationException: SSL authentication error: RemoteCertificateNotAvailable, RemoteCertificateChainErrors
  at System.Net.Mail.SmtpClient.<callback>m__4 (System.Object sender, System.Security.Cryptography.X509Certificates.X509Certificate certificate, System.Security.Cryptography.X509Certificates.X509Chain chain, SslPolicyErrors sslPolicyErrors) [0x00000] in <filename unknown>:0 
  at System.Net.Security.SslStream+<BeginAuthenticateAsClient>c__AnonStorey7.<>m__A (System.Security.Cryptography.X509Certificates.X509Certificate cert, System.Int32[] certErrors) [0x00000] in <filename unknown>:0 
  at Mono.Security.Protocol.Tls.SslClientStream.OnRemoteCertificateValidation (System.Security.Cryptography.X509Certificates.X509Certificate certificate, System.Int32[] errors) [0x00000] in <filename unknown>:0 
  at Mono.Security.Protocol.Tls.SslStreamBase.RaiseRemoteCertificateValidation (System.Security.Cryptography.X509Certificates.X509Certificate certificate, System.Int32[] errors) [0x00000] in <filename unknown>:0 
  at Mono.Security.Protocol.Tls.SslClientStream.RaiseServerCertificateValidation (System.Security.Cryptography.X509Certificates.X509Certificate certificate, System.Int32[] certificateErrors) [0x00000] in <filename unknown>:0 
  at Mono.Security.Protocol.Tls.Handshake.Client.TlsServerCertificate.validateCertificates (Mono.Security.X509.X509CertificateCollection certificates) [0x00000] in <filename unknown>:0 
  at Mono.Security.Protocol.Tls.Handshake.Client.TlsServerCertificate.ProcessAsTls1 () [0x00000] in <filename unknown>:0 
  at Mono.Security.Protocol.Tls.Handshake.HandshakeMessage.Process () [0x00000] in <filename unknown>:0 
  at (wrapper remoting-invoke-with-check) Mono.Security.Protocol.Tls.Handshake.HandshakeMessage:Process ()
  at Mono.Security.Protocol.Tls.ClientRecordProtocol.ProcessHandshakeMessage (Mono.Security.Protocol.Tls.TlsStream handMsg) [0x00000] in <filename unknown>:0 
  at Mono.Security.Protocol.Tls.RecordProtocol.InternalReceiveRecordCallback (IAsyncResult asyncResult) [0x00000] in <filename unknown>:0 
  --- End of inner exception stack trace ---
  at Mono.Security.Protocol.Tls.SslStreamBase.AsyncHandshakeCallback (IAsyncResult asyncResult) [0x00000] in <filename unknown>:0 
  --- End of inner exception stack trace ---
  at System.Net.Mail.SmtpClient.Send (System.Net.Mail.MailMessage message) [0x00000] in <filename unknown>:0 
  at P2Mailclient.SMTPClient.send (P2Mailclient.Email email) [0x00089] in /path/to/my/project/SMTPClient.cs:57 

Does anyone have an idea why I might be getting this error?

Answer 1

Gmail's SMTP server requires you to authenticate your request with a valid gmail email/password combination. You do need SSL enabled as well. Without actually being able to see a dump of all your variables being passed in the best guess I can make is that your Credentials are invalid, make sure you're using a valid GMAIL email/password combination.

You might want to read here for a working example.

EDIT: Okay here's something I wrote and tested just then and it worked fine for me:

public static bool SendGmail(string subject, string content, string[] recipients, string from) {
    if (recipients == null || recipients.Length == 0)
        throw new ArgumentException("recipients");

    var gmailClient = new System.Net.Mail.SmtpClient {
        Host = "smtp.gmail.com",
        Port = 587,
        EnableSsl = true,
        UseDefaultCredentials = false,
        Credentials = new System.Net.NetworkCredential("******", "*****")
    };

    using (var msg = new System.Net.Mail.MailMessage(from, recipients[0], subject, content)) {
        for (int i = 1; i < recipients.Length; i++)
            msg.To.Add(recipients[i]);

        try {
            gmailClient.Send(msg);
            return true;
        }
        catch (Exception) {
            // TODO: Handle the exception
            return false;
        }
    }
}

If you need any more info there's a similar SO article here