You can choose to have your inbox organized by message or by conversation by clicking on the "View" drop down in the top navigation.
When you organize your email by message, emails are displayed with the most recent message listed first.
When you organize your email by conversation, your messages are grouped by the subject.
The subject displays only once in the conversation view pane. The number of messages in the conversation is displayed in parentheses after the subject. When new email comes in, the conversation is moved to the top of your email messages. Conversations containing unread messages are shown in bold.
Double click the conversation to display the messages within the conversation. Click a message to display its contents.
All messages related to the conversation are displayed, even if the messages are stored in other folders. Look to the "Folder" column to see where the message is stored.
Conversations do not continue indefinitely. If the last message in the conversation is 30 days old or older, new messages or replies sent with the same subject begin a new conversation.