If you mean thread then look carefully at the column headers above the first post in your Inbox and/or any other folder where your emails are stored.

If it is the same as my own email software, it is not easy to find or recognise, (it is not that intuitive), but with mine it looks like a very small diagram of steps. Mine is the first header of the column headers, on the left.

If you find it, click it once and 'same messages' should thread. If you can't find it, try Help for Threading, or Threaded messages.

Hope that helps.

Mark