CheckBox disabled in asp.net

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C#

ASP.NET

i am trying to sending mail in asp.net . when admin send mail to respective email id then admin check the check boxes and send the email, after this i want to show check box disabled this is what i want because when admin again login then he will not able to check again in same check boxes because mail already sent and when new email come then check box must be enable because admin still not perform any check in new email ...

In my project..when i check in check boxes and click on submit button then email send but when admin again login and then check whether new record comes or not then previous check box always enabled but i want disabled check boxes in previous record because email already send ..

like this

emalid
john_11@gmail.com   (this is new record then  check box enabled)
kety_45@yahoo.com  ( admin already check in this and mail  send then check box must be
 disabled)

here is code button code

protected void btnSendMail_Click(object sender, EventArgs e)
          {
           string connStr =
           ConfigurationManager.ConnectionStrings["email"].ConnectionString;
           SqlConnection mySQLconnection = new SqlConnection(connStr);
           string empId = string.Empty;
           DataTable dt = new DataTable();
           try
           {
           mySQLconnection.Open();
           for (int i = 0; i < Repeateremail.Items.Count; i++)
           {
            CheckBox checkboc =
          ((CheckBox)Repeateremail.Items[i].FindControl("chkSelect"));
            if (checkboc != null)
            {

                if (checkboc.Checked == true)
                {
                    string emailId =
               ((Label)Repeateremail.Items[i].FindControl("lbl_email")).Text;

                    SendEmailUsingGmail(emailId);
                    dt.Clear();
                    dt.Dispose();
                }
            }
        }


    }