ส่งเมลไม่ได้ error Mailbox unavailable. The server response ช่วยดูที

protected void Button1_Click(object sender, EventArgs e)
        {
            SmtpClient SmtpServer = new SmtpClient("smtp.domain.com");
            MailMessage mail = new MailMessage();
            mail.From = new MailAddress("[email protected]");
            mail.To.Add("[email protected]");
            mail.Subject = "Test Mail - 1";
            mail.IsBodyHtml = true;
            mail.Body = "Mail body";
            SmtpServer.Port = 587;
            SmtpServer.UseDefaultCredentials = false;
            SmtpServer.Credentials = new System.Net.NetworkCredential("[email protected]", "ไม่บอกนะอิอิ");
            SmtpServer.EnableSsl = true;
            SmtpServer.Send(mail);
          }