Conversation View
By default, when a TROY O365 account is added to Outlook, messages are displayed in Conversations. A Conversation includes all messages in the same thread with the same subject line.
The Conversation view can be disabled by choosing Filter above your message list, and then selecting Show As > Messages
Additional information for Conversation View and other customizations for Outlook can be found at https://support.office.com/en-us/article/Customize-dba4d238-f29f-4800-b0cb-504359469d9d#ID0EAABAAA=Customize_view