If your entire mailbox needs cleaning out, the Mailbox Cleanup tool is a better option.
Click the File tab.
Under Mailbox Settings, you can see how much total space you have in your mailbox and how much is used.
Click the Tools button.
Select Mailbox Cleanup.
The Mailbox Cleanup dialog box shows basic information about mailbox usage.
There are a number of tools available here, described in the table at the end of the page.
Use the options in the Mailbox Cleanup dialog box to manage the size of your mailbox.
Click the Close button.
The Mailbox Cleanup dialog box closes, and now your mailbox consumes less space.
Mailbox Cleanup Tools
View Mailbox Size
Clicking this button opens a dialog box that shows the total size of your mailbox, as well as the size of each folder within it.
You can search your mailbox for messages older than a specified number of days or larger than a specified file size. Once found, you can look through the results and delete ones you don’t need.
The AutoArchive command will move old items from your inbox into an archive file stored on your computer. The items will be accessible by browsing the Archive folders in the Folder list.
Empty Deleted Items
You can click the View Deleted Items Size button to see how much space is taken up by your Deleted Items folder, and click the Empty button to clear it out.
Sometimes, a version of an item on your computer and that same item on the server might have a conflict. When that happens, both versions of that item will be saved. Click the View Conflicts Size button to see how much space those take up, and click Delete to delete the conflicted version.