Using New Outlook on Windows for professional communication and productivity
In the new Outlook for Windows, there currently isn't a feature that allows you to keep folders expanded between sessions. Each time you close and reopen Outlook, the folder structure resets to a collapsed state. Unfortunately, this is a limitation of the new Outlook interface compared to the classic version, where you could maintain the expanded state of folders.
If this feature is important to you, consider providing feedback to Microsoft through the Outlook feedback option, as user suggestions can influence future updates.