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

That error message is typically caused by one of the following:

• Incorrect connection settings, such as the wrong port specified for the secured or non-secured connection
• Incorrect credentials. I would verify the username and password combination, to make sure the credentials are correct.

Answer 2

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("from@gmail.com", "From Name");
var toAddress = new MailAddress("to@example.com", "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);
}

Answer 3

If you are sure that your Username and Password are correct, but you still get errors then it means that Gmail has blocked your application.

You should check 'Turn on' radio button in the following link relevant to your gmail account. It would then work fine.