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

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.