How To Check Your Mailbox Usage

In Outlook, you can determine your mailbox usage. You can check for the distribution usage of the various folders.

To know the usage occupied by various folders, select "Tools" -> "Mailbox Cleanup". You should see the screen below.

Click on the View Mailbox Size button. You would see the distribution of your mailbox usage in the screen below.

Alternatively to initiate mailbox cleanup, you can choose to click on the various buttons under "Mailbox Cleanup" such as to find mails that are old than a specific date or larger than a specific size, mails that are occupying the "Deleted Items" folder as well as to setup archive folders.