mail sending error using c# windows forms

MailMessage mail = new MailMessage();

            SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");

            

            mail.From = new MailAddress("[email protected]");

            mail.To.Add("[email protected]");

            mail.Subject = "Test Mail - 1";

            mail.Body = "mail with attachment";

            

            System.Net.Mail.Attachment attachment;

            attachment = new System.Net.Mail.Attachment(@"C:\Users\Engineer\Desktop\1.txt");

            mail.Attachments.Add(attachment);

this is mail sending code it not works 
error message is : The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at

I am using windows 8.1. is this is due to windows ? please help me 

            SmtpServer.Port = 587;

            SmtpServer.UseDefaultCredentials = false;

            SmtpServer.Credentials = new System.Net.NetworkCredential("myeamil.com", "mypassword");

            SmtpServer.EnableSsl = true;

            

            SmtpServer.DeliveryMethod = SmtpDeliveryMethod.Network;

            ServicePointManager.ServerCertificateValidationCallback = delegate(object s, System.Security.Cryptography.X509Certificates.X509Certificate certificate, System.Security.Cryptography.X509Certificates.X509Chain chain,

                                    System.Net.Security.SslPolicyErrors sslPolicyErrors)

            {

                return true;

            };

            SmtpServer.Send(mail);