ช่วยหน่อยครับผมเขียนโปรแกรมส่งอีเมลแต่อีเมลที่ส่งมันตกไปในถังขยะครับต้องแก้ยังไงครับ

        public void send_mail()
        {
            MailMessage mail = new MailMessage();
            mail.From = new MailAddress(textBox1.Text);
            mail.To.Add(textBox3.Text);
            mail.Subject = textBox5.Text;
            mail.Body = textBox4.Text;
            if (textBox6.Text != "")
            {
                mail.Attachments.Add(new Attachment(textBox6.Text));
            }
            SmtpClient smtpServer = new SmtpClient();
            smtpServer.UseDefaultCredentials = true;
            //smtpServer.Credentials = new System.Net.NetworkCredential(textBox1.Text, textBox2.Text);
            smtpServer.Port = 25;
            smtpServer.Host = "xx.xx.xx.xxx";// SMTP เป็นรูปแบบ IP
            smtpServer.EnableSsl = checkBox1.Checked;
            //mail = null;
            try
            {
            System.Net.ServicePointManager.ServerCertificateValidationCallback = delegate (object s,
            System.Security.Cryptography.X509Certificates.X509Certificate certificate,
            System.Security.Cryptography.X509Certificates.X509Chain chain,
            System.Net.Security.SslPolicyErrors sslPolicyErrors)
                {
                    return true;
                };
                smtpServer.Send(mail);
                MessageBox.Show("Mail Send Success Fully");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message,"Information");
            }

        }