System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated

Question

I'm trying to send email with my website's address from a C# application.
This worked fine for several months until recently. (maybe my provider changes some things or someone else changed settings)

Here's the code:

  private void sendEmail(Email invite) {
            MailMessage mail = new MailMessage();
            SmtpClient SmtpServer = new SmtpClient(smtpServerName);
            mail.From = new MailAddress(emailUsername);

            mail.To.Add(invite.RecipientEmail);
            mail.Subject = invite.MessageSubject;
            mail.Body = invite.MessageBody;

            SmtpServer.UseDefaultCredentials = false;
            SmtpServer.Port = 587;
            SmtpServer.Credentials = new System.Net.NetworkCredential(emailUsername, emailPassword);
//          SmtpServer.EnableSsl = true;
            SmtpServer.Send(mail);
        }  

Here's the error:

The SMTP server requires a secure connection or the client was not authenticated. The server response was: SMTP authentication is required.

Looking at other questions I tried what they suggested, to make SmtpServer.EnableSsl = true. This didn't work at all. It gave the following:

System.Net.Mail.SmtpException: Server does not support secure connections.

I'm guessing I should disable SSL and have it the way it was before.

Any suggestions how to make email sending work again?

EDIT
I've tried without SmtpServer.UseDefaultCredentials = false;
I've tried with it set to true: SmtpServer.UseDefaultCredentials =true;
I've tried commenting that line along with the following //SmtpServer.Credentials = new System.Net.NetworkCredential(emailUsername, emailPassword);

Answer 1

I think you have to set DeliveryMethod = SmtpDeliveryMethod.Network

this one is currently working in my PC, just i checked,working nice,try this

using System.Net;
using System.Net.Mail;

var fromAddress = new MailAddress("[email protected]", "From Name");
var toAddress = new MailAddress("[email protected]", "To Name");
const string fromPassword = "fromPassword";
const string subject = "Subject";
const string body = "Body";

var smtp = new SmtpClient
           {
               Host = "smtp.gmail.com",
               Port = 587,
               EnableSsl = true,
               DeliveryMethod = SmtpDeliveryMethod.Network,
               UseDefaultCredentials = false,
               Credentials = new NetworkCredential(fromAddress.Address, fromPassword)
           };
using (var message = new MailMessage(fromAddress, toAddress)
                     {
                         Subject = subject,
                         Body = body
                     })
{
    smtp.Send(message);
}