SMTP Server Error: Bad sequence of commands. The server response was: you must authenticate first (#5.5.1)

This error occurs due to lock down of default mail server (127.0.0.1) by web hosting companies due to SPAM or what ever other security reasons. But if you use “localhost” or default server “127.0.0.1” on your local machine it should not be an issue. But if you have a situation where you need to relay e-mail to a remote mail server that is secured you get this exception.

So you need to handle well using built in classes provided by .NET. This is take care by NetworkCredential Class. This class provides credentials for password-based authentication schemes such as basic, digest, NTLM, and Kerberos authentication.

Here is a fully working quick code sample that you can use to get started on your own SMTP-Authentication supporting e-mail code.

   1: try

   2: {

   3:     SmtpClient smtpclient = new SmtpClient();

   4:     NetworkCredential NetworkCredential = new NetworkCredential(smtpUserName, smtpPassword);

   5:     smtpclient.Credentials = NetworkCredential;

   6:     string MailServer = ConfigurationManager.AppSettings["SMTPServer"].ToString();

   7:     MailMessage objEmail = new MailMessage();

   8:  

   9:     string FromEmail = ConfigurationManager.AppSettings["ErrorReportEmailFrom"].ToString();

  10:     if (FromEmail.IndexOf(";") > 0)

  11:     {

  12:         FromEmail = FromEmail.Replace(";", ",");

  13:     }

  14:     MailAddress FromAddress = new MailAddress(FromEmail);

  15:     objEmail.From = FromAddress;

  16:  

  17:     string ToEmail = ConfigurationManager.AppSettings["ErrorReportEmailTo"].ToString();

  18:     if (ToEmail.IndexOf(";") > 0)

  19:     {

  20:         ToEmail = ToEmail.Replace(";", ",");

  21:     }

  22:     objEmail.To.Add(ToEmail);

  23:  

  24:     string CcEmail = ConfigurationManager.AppSettings["CcEmail"].ToString();

  25:     if (CcEmail.IndexOf(";") > 0)

  26:     {

  27:         CcEmail = CcEmail.Replace(";", ",");

  28:     }

  29:     objEmail.CC.Add(CcEmail);

  30:  

  31:     string BccEmail = ConfigurationManager.AppSettings["BccEmail"].ToString();

  32:     if (BccEmail.IndexOf(";") > 0)

  33:     {

  34:         BccEmail = BccEmail.Replace(";", ",");

  35:     }

  36:     objEmail.Bcc.Add(BccEmail);

  37:  

  38:     objEmail.Subject = strSubject;

  39:     try

  40:     {

  41:         objEmail.Body = strBody;

  42:         objEmail.IsBodyHtml = true;

  43:         smtpclient.Host = MailServer;

  44:         smtpclient.Send(objEmail);

  45:     }

  46:     catch (Exception ex)

  47:     {

  48:         throw ex;

  49:     }

  50:     finally

  51:     {

  52:         objEmail.Dispose();

  53:     }

  54: }

  55: catch (Exception ex)

  56: {

  57:      

  58: }

Enjoy coding!!