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There are now two versions of Outlook for Windows and Mac. These two versions will co-exist for a while as Microsoft adds more features to the new Outlook. The new Outlook for Windows brings the latest features, intelligent assisted capabilities and a new modern and simplified design to your Outlook app. You can tailor it to your style and do more with new Outlook. Below are some links with more information and instructions to enable or disable the new Outlook.
Outlook for Windows
Outlook for Mac