Normally when you look at your Mail, you are looking at it in chronological order from newest to oldest. (At least that is how I view my mail.)
(If you aren't seeing your mail in chronological order, make sure that you have selected Date Received as your sort order.)
Mail has another way to view your email lists and that is called Threaded. Threaded view looks for messages that have the same sender and the same subject and groups them together. This can be helpful if you have received messages in the same conversation (or "thread") over time.
To change to Threaded view, go to the View menu on the Menu Bar and choose Organize by Thread.
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