What’s New in OWA on Exchange 2010 | Viewing Email Messages by Conversation
  
Viewing Email Messages by Conversation
When you access OWA the first time, you will notice that your email messages are arranged by conversation. When the Conversations view is turned on, messages that share the same subject display as a consolidated thread that can be expanded or collapsed.
Expanding Conversations
Grouped email messages that share the same subject are indicated by an arrow to the left of the most recent message.
Step 1: Click the arrow to expand the conversation.
Step 2: All messages with the selected subject display within the conversation. Click a message to view its content.
Turning On/Off Conversation View
Above the email list, click View and enable or disable Use Conversations.