Send emil using yahoo account and gamil from C# winform

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C#

hi every one i make an application that send Email from yahoo mail to gmail and yahoo mail.
when i send emil using my gamil account it work fine but when i try to send emil using my yahoo mail account it give an error please any one help me.
when i use this code to send emil using my yahoo account it give me error
error message is follwoing
"

sending system.net.mail.smptException mailbox unavilable. the server response was requested mail action not taken mailbox unavilable

"

What I have tried:

Im using this code

private void SmptTypecomboBox1_SelectedIndexChanged(object sender, EventArgs e)
       {
           if (SmptTypecomboBox1.SelectedIndex == 0)
           {
               portno =587;
               servicetype = "smtp.gmail.com";
           }
           if (SmptTypecomboBox1.SelectedIndex == 1)
           {
               portno = 587;
               servicetype = "smtp.mail.yahoo.com";
           }

       }

private void Sendbutton1_Click(object sender, EventArgs e)
        {
            using (OpenFileDialog attachement = new OpenFileDialog()
            {
                Filter = "Pdf Files|*.pdf",
            ValidateNames = true
            })
            {
                if (attachement.ShowDialog() == DialogResult.OK)
                {
                    if (Isvalid())
                    {
                        Send(FromtextBox.Text, paswordtextBox.Text, TotextBox.Text, MessagetextBox.Text, AttachmenttextBox6.Text, servicetype, portno, attachement.FileName);
                    }
                }
            }
           // clearAllfileds();
        }

public void Send(string from, string password, string to, string Message, string subject, string host, int port, string file)
       {

           MailMessage email = new MailMessage();
           email.From = new MailAddress(from);
           email.To.Add(to);
           email.Subject = subject;
           email.Body = Message;
           SmtpClient smtp = new SmtpClient(host, port);
           smtp.UseDefaultCredentials = false;
           NetworkCredential nc = new NetworkCredential(from, password);
           smtp.Credentials = nc;
           smtp.EnableSsl = true;
           email.IsBodyHtml = true;
           email.Priority = MailPriority.Normal;
           email.BodyEncoding = Encoding.UTF8;

           if (file.Length > 0)
           {
               Attachment attachment;
               attachment = new Attachment(file);
               email.Attachments.Add(attachment);
           }

           // smtp.Send(email);
           smtp.SendCompleted += new SendCompletedEventHandler(SendCompletedCallBack);
           string userstate = "sending ...";
           smtp.SendAsync(email, userstate);
       }
       private static void SendCompletedCallBack(object sender, AsyncCompletedEventArgs e)
       {
           string result = "";
           if (e.Cancelled)
           {

               MessageBox.Show(string.Format("{0} send canceled.", e.UserState), "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
           }
           else if (e.Error != null)
           {
               MessageBox.Show(string.Format("{0} {1}", e.UserState, e.Error), "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
           }
           else
           {
               MessageBox.Show("your message is send", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
           }
       }