Hi @Eaven HUANG ,
Welcome to our forum!
As I know, the X icon is used to close Outlook window. I'm afraid that we cannot use it to minimize Outlook window. It's by design.
On the other hand, we can use the following option to minimize Outlook window. And for more information, please refer to: Minimize Outlook.
Hope to help you!
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.