What is the best way to sending mail from gridview check box checked row

I am using gridview and it contains checkboxes.

when i click submit button, i am sending mail to the customer for every checking record.

if i save record without mail, data will be saving more accurate and fast.

But, when i send mail for every records from the gridview, it's taking long time to process it and done.

I know the reason, it will hit smtp server every records and return back is taking time in the server.

What is the alternative way to sending mail while submit the records...

below is my complete code for reference.

 protected void Submit_Click(object sender, EventArgs e)
        {
            try
            { 		
		System.Threading.Thread.Sleep(3000);
                    //After Setting Page Index Loop through its Rows
                    foreach (GridViewRow gvrow in gbCustomer.Rows)
                    {
                        if (gvrow.RowType == DataControlRowType.DataRow)
                        {
                            var checkbox = gvrow.FindControl("CheckBox1") as CheckBox;
                            if (checkbox.Checked)
                            {
				var Id = (gvrow.FindControl("hdnctrlId") as HiddenField).Value;
                                var CustNo = (gvrow.FindControl("hdnCustNo") as HiddenField).Value;
                                var CustName = (gvrow.FindControl("hdnCustName") as HiddenField).Value;
                                var DOJ = (gvrow.FindControl("hdnDOJ") as HiddenField).Value;
                                //********* Sending Mail
                                SendMail(Id, CustNo, CustName, DOJ);
                                //********* Sending Mail
                             
                                CheckBox chk = (CheckBox)gvrow.FindControl("CheckBox1");
                                chk.Checked = false;
                                CheckBox chkAll = (CheckBox)gbCustomer.HeaderRow.FindControl("chkAll");
                                chkAll.Checked = false;
                            }
                        }
                    }
                ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "alert", "alert('Mail Sent successfully.!');window.location.href='NewCustomer.aspx';", true);
	     }
            catch (Exception ex)
            {
                ErrorMessage.Text = ex.Message;
                ErrorMessage.Visible = true;
            }
            finally
            {
                
            }
        }
  private void SendMail(string Id, string CustNo, string CustName, string DOJ)
        {
            try
            {
                FromEmail = "******@biech.com";
                ToEmail = "******@biech.com";
                if (!string.IsNullOrEmpty(ToEmail))
                {
                    MailMessage mm = new MailMessage(FromEmail, ToEmail);
                    mm.Subject = "Reg : Customer Request";
                    string strhtml = "<HTML>";
                    strhtml += "<body>";
                    strhtml += "<br>";
                    strhtml += "<table>";
                    strhtml += "<tr>";
                    strhtml += "<td>";
                    strhtml += "Cust. Name ";
                    strhtml += "</td>";
                    strhtml += "<td>";
                    strhtml += "DOJ. ";
                    strhtml += "</td>";
                    strhtml += "<tr><td>";
                    strhtml += "&nbsp;" + CustName + "  </td>";
                    strhtml += "<td>";
                    strhtml += "&nbsp;" + DOJ + "  </td>";
                    strhtml += "<br>";
                    strhtml += "<div align='left'>Regards,</div>";
                    strhtml += "<div align='left'>Alex Paul</div>";
                    strhtml += "</body>";
                    strhtml += "</html>";
                    mm.Body = strhtml;
                    mm.IsBodyHtml = true;
                    SmtpClient smtp = new SmtpClient();
                    smtp.Send(mm);
                }
            }
            catch (Exception ex)
            {
                Response.Write("Error occurred: " + ex.Message);
            }
        }