By default, Mail for Windows 10 groups messages by conversation. To un-group messages, and instead to list them one at a time in chronological order, follow these steps:
From the Inbox pane, press the Tab key until you reach the Settings button and then press Enter.
Press the Down Arrow key until you reach the Reading settings and then press Enter.
Press the Tab key to go to the Show messages arranged by conversation toggle switch, and press Enter to turn it off.
Press the Esc key to close Reading settings, and then press Esc again to close the Settings pane.
Focus returns to the Inbox message list.