How to add item to listbox and then send email.

Question

Good day guys, I'm a semi-intermediate here in c#. Actually, the said topic name it won't work with my codes implemented, i need some help on how to improve my timer tick procedure code as shown what I've tried below.

private void timer1_Tick(object sender, EventArgs e)
      {
          statmessage();

          int i = listBox1.SelectedItems.Count + 1;
          do

          {
              var exp = (EItem)listBox1.Items[i];
              var timeVisible = DateTime.Now - exp.Added;
              if (timeVisible.TotalSeconds > 5)
                  if (counter == 100)
                  {

                      SendMessage();
                      counter = counter - 1;
                      ++i;
                  }
                  else if (counter <= 0)
                  {

                      statmessage();
                  }

          } while (i < -1);
          }

      }
  }

Your feedback is highly appreciated and will help to improve the said concern, Thank you very much.