Well I read messages in the lower pane in the main window, so in a sense I never leave the Inbox, or whatever folder the message is in. You are presumably opening your messages in separate windows or in tabs. In both cases the answer is to close the window or tab. Esc will close the window, ctrl+w will close the Tab. Both can be closed by clicking on the X in the top corner, in the case of a window, and on the tab's handle in a tab. Some users like to just move to the next unread message using a keystroke or toolbar button. I don't do that; I much prefer to see where I'm going. It should always be possible to go directly to the Inbox or current folder by selecting it at the left, in a narrow pane listing accounts and folders, or by clicking on its tab at the top. Can you not see a list of accounts and folders?