It's a good idea to empty your deleted items. You may be running out of space in your mailbox. If it's in deleted items, you probably don't want it so why keep it in your mailbox.
Here are the easy steps:
Outlook is already up.
Click on File, then Options, then Advanced.
Under Outlook Start and Exit, check the box to Empty deleted items when exiting Outlook. Then Ok.
When you are finished with Outlook, use File, then Exit.
Click on Yes to Empty.
Note: If you hit the X in upper right, it won't try to empty deleted items