The proper way to remove mail from your email account and reduce its gigabyte usage is to move mail into the Trash folder. Both the Trash Folder and the Junk folders empty themselves of any email placed there for thirty days but you can manually empty those folders by clicking the folder and then the gear wheel at the bottom of the window and choose empty.
Remove Items Marked For Deletion In Outlook
If you have emptied mail from both the Trash and Junk folders in Webmail but still see items marked for deletion in Outlook and wish to reduce the size of your mailbox, then click the gear wheel to reveal more options and select Compact. Depending on the amount of items it may take several minutes to completely delete all items from the mail server and see the reduced inbox size.
Using Outlook to Purge Items Marked For Deletion
Click here for Outlook 2007 Instructions
Using Outlook 2010 on a Mac to Purge Items Marked For Deletion
Using Mac Mail to Purge Items Marked For Deletion
1) Goto your Inbox
2) Select 'Mailbox' from the top navigation
3) Select 'Erase Deleted Items' then 'In All Accounts'
4) Log into webmail.com and verify your usage percent has dropped. (pictured below)