A OTP porgram getting error (Failure sending mail)

Question

I'm making a User Registration System (In the login section there is another part called (forgot password) So when user click on forgot password (he/she should provide a gmail account in which OTP will be send but I'm getting the error (Failure sending mail) ))

private void button2_Click(object sender, EventArgs e) { string from, pass, msgbody;

        Random rand = new Random();
        randomCode = (rand.Next(999999)).ToString();

        MailMessage mm = new MailMessage();
        to = (textBox1.Text).ToString();
        from = "**************"; //here it is the email of the sender
        pass = "**********";// password of the sender
        msgbody = "Reset Code "+ randomCode;
        mm.To.Add(to);
        mm.From = new MailAddress(from);
        mm.Body = msgbody;
        mm.Subject = "Password reset code";

        SmtpClient smtp = new SmtpClient("smtp.gmail.com");
        smtp.EnableSsl = true;
        smtp.Port = 587;
        smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
        smtp.Credentials = new NetworkCredential(from,pass);
       try
        {
            smtp.Send(mm);
            MessageBox.Show("Code sent successfully");
        }
        catch(Exception ex)
        {
            MessageBox.Show(ex.Message,"Error",MessageBoxButtons.OK,MessageBoxIcon.Error);
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        if(randomCode == (textBox2.Text).ToString())
        {
            to = textBox1.Text;
            Form9 f9 = new Form9();
            f9.Show();
            this.Hide();
        }
        else
        {
            MessageBox.Show("Wrong code","Error",MessageBoxButtons.OK,MessageBoxIcon.Error);
        }
    }