Messages in an email system can be set to show the full thread of replies with the ability to then click into prior messages from within the newer messages. This is called conversation view and seems to be the most popular way to show email because this is the default in most email systems including Outlook. Now, if you like conversation view you can stop reading.
HOWEVER, THIS VIEW DRIVES ME ABSOLUTELY BONKERS.
I prefer the message history to display fully as plain text. Then you do not need to click to see complete messages. You just need to scroll.
I had searched without success for how to change this because I was using the search term threaded. Today I discovered that this is called the conversation view in Outlook. To change this you just need to find where the setting is for your mail viewer.
OLD: For the Outlook browser application in your inbox (or any other mail box) select filter. Then hover over show as and you will have the choice to select Messages, or Conversations.
Update for 2021 – it looks like the menu above has changed. Find this in the general settings:
As usual your browser may not have such pleasant shades of green showing depending on your other settings.
Once I knew what to search terms to use I could find other sites with instructions for the desktop app. “how to turn off conversation view in outlook” turns up some good ones.