How Do I Turn Off Arrange By. Yur2die4 is correct - in other words the Arrangement view plays by default unless some clip triggered in Session view overrides it. Go to View and make sure that the Auto arrange option is unchecked.

There are some people in our company used to the old fashion way of seeing their emails not sort by thread. If the view type is on list or details you wont be able to disable auto-arrange. If you are using Outlook 2007 please click View Arrange By.
Click Mail in the navigation pane.
And then uncheck the Show in Groups. You can also process the other changes into the Word using the Track Changes options and the other choices from the dropdown. Uncheck Automatically group according to arrangement and Ok your way back to the view. View- Arrange By- disable.