Checking Mailbox Quota

1. Open your Microsoft Outlook

2. Click on File > Info > Cleanup Tools > Mailbox Cleanup...

3. Click on View Mailbox Size...

4. You will receive a similar result as below. The Total size (including subfolders) will indicate the existing space taken up by your emails. If you refer to the table content below, it shows you the size of each individual folders in your mailbox thus you will have an idea of which are the folders to cleanup.