To minimize Outlook to the system tray when minimized, you can right-click on the Outlook icon on the taskbar and select Minimize to System Tray. On Windows 11, this option seemed to be missing. So to configure this behavior, the following registry modification can be made.
Note: This is for Outlook 2021 (16.0). You may need to adjust if you’re using Outlook 2019 (15.0) etc.
reg add HKCU\Software\Microsoft\Office\16.0\Outlook\Preferences /v MinToTray /t REG_DWORD /d 1