When you open a message in Outlook to reply to it or forward it, the original message window remains open, so you have to close it manually afterward. For those who are bothered by this small inconvenience, you can simply change it.
Click on the File menu, select Preferences, and select the Mail category from the left-hand column. Scroll down to the Replies and Forwarded Mail area, tick the option Close original message window if replying or forwarding, and save the change by clicking OK.