When you receive emails which have same subject, Gmail groups them in a single thread to give you conversational view.

In the conversation thread, most recent email is at the top, though its a good feature but because of any reasons if you do not like this feature, and want simple old style mailbox, then you are in luck, you can simply turn it off as I show in this video.

Step 1: Login to Gmail

Step 2: Go to Gmail Settings

Step 3: Under General Tab, look for “Conversation View”

Step 4: Click on “Conversation View Off”

Now your threads should be separated

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