Microsoft has been lurking about the idea of placing the Outlook navigation bar to the left side for quite a while and has finally pulled the trigger lately. If you are one of those who, I honestly have lost count of how many, genuinely hate this new look, here is the solution for you.
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Go to File > Options, switch to the Advanced tab and uncheck the option called Show Apps in Outlook.
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Click OK to save the setting. Restart Outlook and you will be back to classic mode.
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There is also a registry hack you can use to trick Outlook as well.
Press Win + R, type Regedit and press Enter to open the Registry Editor.
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Navigate to the following key,
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\ExperimentEcs\Overrides
And set the following value to false.
Microsoft.Office.Outlook.Hub.HubBar
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Obviously, the native setting option is always recommended but the registry option does give SysAdmins a bit more options to manage a large group of computers. You can export the key and import them as you wish. You can also use the PowerShell script to remotely push out the change as well.