Outlook ought to show you how full your mailbox is by default. Unfortunately, it doesn’t. Here’s how you can tell Outlook to show your mailbox quota all the time.

• Right-click on the status bar at the bottom of Outlook 2010 or Outlook 2013. (It might work in other versions as well.)
• Select “Quota Information.”

Adding your mailbox quota information to Outlook’s status bar.

Now you will be able to see how much free space you have in your mailbox down in the bottom left corner in the status bar. Don’t expect it to show changes instantly. It can take a little time after you delete a lot of email for the change in size to show up.