To check the storage usage of your Outlook.com mailbox account and free up space, follow these steps: 1. Open Outlook.com and sign in with your credentials. 2. Click on the Settings (gear) button located in the top-right corner. 3. Select the 'View all Outlook settings' option at the bottom. 4. Click on General. 5. Click on Storage. 6. You will be able to see the storage usage for your inbox, sent items, and deleted items in Outlook.com. 7. If you want to free up space, you can use the Empty option to remove deleted, sent, archive, and inbox items. 8. Once you complete these steps, the settings page will display your storage usage and how much space is left in your account.