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By default, Outlook keeps a message window open after you reply to or forward that message, meaning you have to manually it when you’re done. You can change that so that Outlook automatically closes the original message window as soon as you hit the “Send” button. Here’s how.

Source: https://www.howtogeek.com/361872/how-to-make-outlook-close-the-original-message-after-replying-or-forwarding-in-office-2016/