Conversation View. One of the more annoying aspects of the Mail app (well, besides it just not being trustworthy), can now be customized. If you've received the updated Mail app from the Windows Store recently, this feature is now available.
Conversation View is where the Mail app will pull all related emails and shove them into a single thread. For some, this could be valuable to go back through the history of a conversation – but, for most, it's just a tedious annoyance.
To get to it…
In the Mail App, tap or click the Settings icon (gear image) at the bottom.
Once the right-pane flies out, choose Options.
Locate the Show messages arranged by conversation and make your choice (off or on).
This setting is configurable for each email account you have setup in the Mail app.