Open your Mailbox
In the upper right corner, you will find a View menu, be sure Messages has been selected.
If you have files you need to keep, save them to your hard drive. Double click on the message, right click on the file and click Save As to save to your CPU.
In your Inbox, to delete a page at a time, click once on the top email, hold the shift key and click once on the email at the bottom of the screen. This will highlight the page. Click the X and wait, this may take a few minutes, do this until there are no files then click the trash can. Do the same thing with Drafts and Sent Items under Folders in the Menu on the left.