Refer below code.
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindGrid();
    }
}
private void BindGrid()
{
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT VEND_EMP, VEN_EMP_NAME, VEN_EMP_PHNO, VEN_EMP_EMAIL FROM Customers"))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                using (DataTable dt = new DataTable())
                {
                    sda.Fill(dt);
                    GridView1.DataSource = dt;
                    GridView1.DataBind();
                }
            }
        }
    }
}
protected void OnPageIndexChanging(object sender, GridViewPageEventArgs e)
{
    GridView1.PageIndex = e.NewPageIndex;
    this.BindGrid();
}
protected void chkSelectAll_CheckedChanged(object sender, EventArgs e)
{
    CheckBox chkAll = (CheckBox)GridView1.HeaderRow.FindControl("chkSelectAll");
    foreach (GridViewRow gvRow in GridView1.Rows)
    {
        CheckBox chkSel = (CheckBox)gvRow.FindControl("chkSelect");
        if (
            chkAll.Checked)
        {
            chkSel.Checked = true;
        }
        else
        {
            chkSel.Checked = false;
        }
    }
}
protected void Button2_Click(object sender, EventArgs e)
{
    foreach (GridViewRow gvRow in GridView1.Rows)
    {
        CheckBox chkSel = (CheckBox)gvRow.FindControl("chkSelect");
        if (chkSel.Checked)
        {
            try
            {
                string empName = gvRow.Cells[1].Text;
                string phoneNo = gvRow.Cells[2].Text;
                string message = TextArea1.Text;
                string SMSurl = "MY API";
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(SMSurl);
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                StreamReader sr = new StreamReader(response.GetResponseStream());
                ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('Msg Successfully Send...')", true);
            }
            catch (Exception ex)
            {
                ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('Msg Sending Failed...')", true);
            }
        }
    }
}
protected void Button3_Click(object sender, EventArgs e)
{
    //Create a temporary DataTable
    DataTable dtCustomers = new DataTable();
    dtCustomers.Columns.AddRange(new DataColumn[2] 
    {
        new DataColumn("VEN_EMP_EMAIL", typeof(string)),
        new DataColumn("VEN_EMP_NAME",typeof(string))
    });
    //Copy the Checked Rows to DataTable
    foreach (GridViewRow row in GridView1.Rows)
    {
        if ((row.FindControl("chkSelect") as CheckBox).Checked)
        {
            dtCustomers.Rows.Add(row.Cells[3].Text, row.Cells[1].Text);
        }
    }
    string subject = "Welcome Email";
    string body = "Hello {0},<br /><br />Welcome to ASPSnippets<br /><br />Thanks.";
    //Using Parallel Multi-Threading send multiple bulk email.
    Parallel.ForEach(dtCustomers.AsEnumerable(), row =>
    {
        SendEmail(row["VEN_EMP_EMAIL"].ToString(), subject, string.Format(body, row["VEN_EMP_NAME"]));
    });
}
private bool SendEmail(string recipient, string subject, string body)
{
    MailMessage mm = new MailMessage("indradeo7306@gmail.com", recipient);
    mm.Subject = subject;
    mm.Body = body;
    mm.IsBodyHtml = true;
    SmtpClient smtp = new SmtpClient();
    smtp.Host = "smtp.gmail.com";
    smtp.EnableSsl = true;
    NetworkCredential NetworkCred = new NetworkCredential();
    NetworkCred.UserName = "indradeo7306@gmail.com";
    NetworkCred.Password = "9667044472";
    smtp.UseDefaultCredentials = true;
    smtp.Credentials = NetworkCred;
    smtp.Port = 587;
    smtp.Send(mm);
    return true;
}
protected void Button4_Click(object sender, EventArgs e)
{
    foreach (GridViewRow gvRow in GridView1.Rows)
    {
        CheckBox chkSel = (CheckBox)gvRow.FindControl("chkSelect");
        if (chkSel.Checked)
        {
            string empName = gvRow.Cells[1].Text;
            string phoneNo = gvRow.Cells[2].Text;
            string message = TextArea1.Text;
            // Code to send WhatsAPP message with the details.
            // WhatsAPP API code.
        }
    }
}