How to send email for selected record in Grid view using check box column?

<script language="javascript" type="text/javascript">
        function SelectAll(id) {
            var frm = document.forms[0];
            for (i = 0; i < frm.elements.length; i++) {
                if (frm.elements[i].type == "checkbox") {
                    frm.elements[i].checked = document.getElementById(id).checked;
                }
            }
        }
    </script> 

     protected void Button1_Click(object sender, EventArgs e)
    {
        if (TextBox1.Text == "")
        {
            Page.RegisterStartupScript("Alert Message", "<script language='javascript'>alert('Enter Message in Text Box');</script>");
            return;
        }

        //Checkther whether atleast one check box is selected or not
        for(int i=0;i<=GridView1.Rows.Count-1;i++)
        {           
            GridViewRow row = GridView1.Rows[i];
            CheckBox Ckbox = (CheckBox)row.FindControl("CheckBox1");
            if (Ckbox.Checked == true)
            {
                k++;
            } 
        }

        if (k == 0)
        {
            Page.RegisterStartupScript("Alert Message", "<script language='javascript'>alert('Select atleast one Check box further to proceed');</script>");
            return;
        }

        for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
        {         
            string toemail = GridView1.Rows[i].Cells[3].Text;           
            GridViewRow row = GridView1.Rows[i];
            CheckBox Ckbox = (CheckBox)row.FindControl("CheckBox1");
            if (Ckbox.Checked == true)
            {
                sendMail(toemail);
            }
        }
        TextBox1.Text = "";
    }

    void sendMail(String toemail)
    { 
     try
        {
            //Below I mention From id(Gmail), To mail ID, Subject of the mail, Mail Message
            MailMessage msg = new MailMessage("fromid@gmail.com", toemail , "Testing Mail", TextBox1.Text);
            SmtpClient mailClient = new SmtpClient("smtp.gmail.com", 587);
//Change your gmail user id and password for send email
            NetworkCredential NetCrd = new NetworkCredential("fromid@gmail.com", "fromidpassword");
            mailClient.UseDefaultCredentials = false;
            mailClient.Credentials = NetCrd;
            mailClient.EnableSsl = true;
            mailClient.DeliveryMethod = SmtpDeliveryMethod.Network;
            mailClient.Send(msg);
        }
        catch (Exception ex)
        {
            Response.Write(ex.ToString());
        }
    }