Unable to read data from the transport connection an existing connection was forcibly closed by remote host

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C#

I have developed c# programe to send email alongwith pdf file attachement. when I run the programe it producs the error "unable to read data from the transport connection an existing connection was forcibly closed by remote host". I searched help of this error on internet but could not resolve the issue. I have also on the less secure option of gmail account. Please suggest any remedy.

What I have tried:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Net;
using System.Net.Mail;
using System.Web;

namespace MailMessaje
{
    public partial class Form1 : Form
    {
               
        public Form1()
        {
            InitializeComponent();
        }
        
        private void btnSend_Click(object sender, EventArgs e)
        {
            try
            {
                MailMessage mail = new MailMessage();
                SmtpClient client = new SmtpClient();
                mail.From = new MailAddress("<email>");
                mail.To.Add(this.txtToMailAddress.Text);
                mail.Subject = this.txtSubjectMail.Text;
                mail.Body = this.txtBodyMail.Text;
                System.Net.Mail.Attachment attachFile = new Attachment("D:\\exp1.pdf");
                mail.Attachments.Add(attachFile);

                client.Port = 587;
               
                client.Host = "smtp.gmail.com";
                client.DeliveryMethod = SmtpDeliveryMethod.Network;
                client.EnableSsl = true;
                client.Credentials = new System.Net.NetworkCredential("<email>", "<password>");
                System.Net.ServicePointManager.Expect100Continue = false;
                client.Send(mail);

                MessageBox.Show("Message Send Successfully");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.InnerException.Message);
            }
        }
}}