To reduce Mailbox size, you have to empty or purge items in your mailbox. Simply deleting items does not remove them from the storage database. These items are moved to Trash, where you can recover them if needed. Once an item is “emptied” or “purged”, it is unrecoverable by you as a user.
Options available to you to reduce Mailbox size are explained below:
Empty Trash: This is an option directly from the navigation pane. Simply right-click the Trash icon and click Empty Trash.
Delete and Empty: This is a right-click or toolbar option when selecting one or more items in the item list. Instead of sending these items to Trash, they are deleted and purged at the same time.
For more information about deleting, emptying, and purging Mailbox items, see Deleting Items and Emptying Trash.
Delete from Mailbox Storage Size: The Delete option is available when items are selected in Mailbox Storage Size. When Delete is executed here, it is a a “delete and empty” action, meaning those items are no longer recoverable.
To delete items in Mailbox Storage Size:
Click an item-type button: Trash, Sent, Received, or Personal/Draft.
(Optional) Click the Size header if you want to reverse the sorting of items.
Select one or more items in the data table and click Delete at the bottom of the dialog box.
NOTE:Mailbox Storage Size sorts differently than a folder list, so you may not be able to see all items that you want to delete. In this scenario, you may need to look for, and/or filter, items in the folder list and purge some items from there.