In Windows 11, there are 2 places where you can define which icons will be visible in the system tray.
First place is "Notification settings" accessible by right-clicking on the date in the lower right corner.
Unfortunately, it is not enough to enable Outlook there.
Second place is "Taskbar Settings".
Click Start – Settings – Personalization – Taskbar – Other system tray icons
There are 2 icons in the list named "Microsoft Outlook".
Find and enable "Microsoft Outlook" with an envelope icon.