I usually edit the Windows Registry to "tweak" MS Outlook (Classic) to work better with SmarterMail and not give precedence to MS365.
These are the settings I use (I do this for all users, but I still recommend you look at them to understand what they do before using them. Once you know what they do, you can also decide to use only the ones you are interested in and not all of them.):
reg add HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover /t REG_DWORD /v ExcludeHttpsAutoDiscoverDomain /d 1 /f
reg add HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover /t REG_DWORD /v ExcludeHttpsRootDomain /d 1 /f
reg add HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover /t REG_DWORD /v ExcludeLastKnownGoodURL /d 1 /f
reg add HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover /t REG_DWORD /v ExcludeExplicitO365Endpoint /d 1 /f
reg add HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\RSS /t REG_DWORD /v Disable /d 1 /f
reg add HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences /t REG_DWORD /v DisableTNEF /d 1 /f
reg add HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences /t REG_DWORD /v ExcludeScpLookup /d 1 /f
reg add HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\setup /t REG_DWORD /v DisableOffice365SimplifiedAccountCreation /d 1 /f
Gabriele Maoret - Head of SysAdmins and CISO at SERSIS
Currently manages 6 SmarterMail installations (1 in the cloud for SERSIS which provides services to a few hundred third-party email domains + 5 on-premise for customers who prefer to have their mail server in-house)