However, if the “New Outlook” button disappeared, try running Regedit.exe and go here (or the same with 15.0 if you don’t have 16.0):
Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Preferences
Create DWORD Called UseNewOutlook
Set to 0 (decimal)
Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\General
REG_DWORD “HideNewOutlookToggle”
Set to 1 (decimal)