I'm trying to send emails using gmail's username and password in a Windows application. However, the following code is sending the mail to only the first email address when I collect multiple email address in my StringBuilder instance.

```
var fromAddress = new MailAddress(username, DefaultSender);
var toAddress = new MailAddress(builder.ToString());//builder reference having multiple email address

string subject = txtSubject.Text;
string body = txtBody.Text; ;
var smtp = new SmtpClient
{
    Host = HostName,
    Port = 587,
    EnableSsl = true,
    DeliveryMethod = SmtpDeliveryMethod.Network,
    UseDefaultCredentials = false,
    Credentials = new NetworkCredential(username, password),
    //Timeout = 1000000

};
var message = new MailMessage(fromAddress, toAddress)
{
    Subject = subject,
    Body = body,
    IsBodyHtml = chkHtmlBody.Checked

};

if (System.IO.File.Exists(txtAttechments.Text))
{
    System.Net.Mail.Attachment attechment = new  Attachment(txtAttechments.Text);
    message.Attachments.Add(attechment);
}

if(this.Enabled)
    this.Enabled = false;

smtp.Send(message);

```

What am I doing wrong, and how can I sort out my problem?