That's called "threading". View|Folders|Unthreaded. And you'll probably want to set "sort by date" in the same menu while you're about it. There is a shortcut. Each column in the message list has a thing at the top which looks like a label but is in fact a button. Windows, in particular, styles these in such a way as to mask the fact that they are buttons. Clicking on one of these buttons will, in general, sort by the contents of that column. So click on Date and it'll sort by date. Click again to sort in the opposite direction. The button on the thread column doesn't sort, but switches threading on and off. Threading is useful if you want to see the answers to posts. Sorting by date is also useful. I frequently switch between different views and sortings.