If you select Open in an Existing Message window, the first message that you double click to open will always open an new window. If you leave that window open and double click a second message it will open the new message in the existing message window. If you select Open in a New Window, every message that you double click will open a new window. If you do not close the windows after reading the messages you will be right where you are when you neglect to close the tab after reading the messages. You will have every message that you have read open in a tab and the Inbox tab gets pushed off the screen. '''Close the tab or window after reading the message.''' If you have lots of tabs open you can '''right click''' one of them and select '''Close Other Tabs''' and then use the x on the last tab to close it. This will also close the tab with the Lightning calendar in it so you will need to reopen that. Have you tried using the Message Pane to read the messages and not open them at all? F8 will toggle the Message Pane on and off.