With this new email format, I can't find the "close" button when looking at my emails.
Yes, there is no "Close" button. Use Back.
If your Inbox is in Vertical or Horizontal view when you double-click an email and it will open up in its own window and you will be able to use an x to close it.
(You can also use this method to have multiple emails open at once. Just open an email, click mail, then open a different email.)