C# SMTP authentication failed but credentials are correct

Question

Here is my problem: I wrote whe following program to test if I can send email:

class Program
{
    static void Main(string[] args)
    {
        try {
                   


        

Answer 1

While I had encountered the same issue, I found a solution to the issue here

In short, you must use

smtp.UseDefaultCredentials = false;

AFTER

smtp.Credentials = new NetworkCredential(ConfigurationManager.AppSettings["email.username"].ToString(), 
                                         ConfigurationManager.AppSettings["email.password"].ToString());

So quick summary, something that would look more like this should work:

        SmtpClient smtp = new SmtpClient();
        smtp.Host = ConfigurationManager.AppSettings["email.smtp"].ToString();
        smtp.Port = Convert.ToInt32(ConfigurationManager.AppSettings["email.port"]);
        smtp.Credentials = new NetworkCredential(ConfigurationManager.AppSettings["email.username"].ToString(), 
                                                 ConfigurationManager.AppSettings["email.password"].ToString());
        smtp.UseDefaultCredentials = false;
        smtp.EnableSsl = Convert.ToBoolean(ConfigurationManager.AppSettings["email.enablessl"]);
        Console.WriteLine("Sending email..." + (i+1));
        smtp.Send(mail);

Answer 2

For anyone interested. I had a problem t=with this today. and found my solution was the order.

SMTPClient client = new SMTPClient();
client.UseDefaultCredentials = false; // first after declare and needs to be false.

Then:

client.Credentials = new System.Net.NetworkCredential(sMTPSettings.SMTPLogin, sMTPSettings.SMTPPassword);

Then the rest.

Hope it helps someone

Answer 3

This link saved my life: link

here the problem I experienced is well described: even though the user name is passed with the AUTH LOGIN the server responds with the AUTH_LOGIN_Username_Challenge again.

This problem happens only using System.Net.Mail. The link suggests tre possible solutions:

1)Use CDOSYS (Does not send the User Name with the AUTH Login)
2)Use System.Web.Mail (Does not send the User Name with the AUTH Login)
3)Contact the SMTP Server owner and have them fix the server.

Unfortunately I couldn't the SMTP server owner, so I had to use System.Web.Mail. I know it is deprecated but unfortunately in cases like this there is no other choice IMO.

That's my working code:

System.Web.Mail.MailMessage msg = new System.Web.Mail.MailMessage();
        msg.Body = message.Body;

        string smtpServer = "mail.business.it";
        string userName = "username";
        string password = "password";
        int cdoBasic = 1;
        int cdoSendUsingPort = 2;
        if (userName.Length > 0)
        {
            msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserver", smtpServer);
            msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserverport", 25);
            msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusing", cdoSendUsingPort);
            msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", cdoBasic);
            msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", userName);
            msg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", password);
        }
        msg.To = message.Destination;
        msg.From = "me@domain.it";
        msg.Subject = message.Subject;
        msg.BodyFormat = MailFormat.Html;//System.Text.Encoding.UTF8;
        SmtpMail.SmtpServer = smtpServer;
        SmtpMail.Send(msg);

This answer helped me in using System.Web.Mail.