By default, Outlook Web App displays your messages in Conversion View, which clusters your messages based on Subject, as shown in the example below:

Conversation View Image

Conversation view can help by giving you a compact record of an entire thread, but it can be confusing if you have more than one conversation with identical Subjects, or if you prefer to see your messages in strict chronological order.

If you delete a message while in Conversation view, you actually delete all messages in that thread.

To turn off Conversation view, click Conversations by (just under the search box), then, under Conversations, select off.