In outlook, right click on the folder you want to auto delete emails and select Properties.

Click the AutoArchive tab, and select the "Archive this folder using these settings:," select how frequently you want to auto delete the contents of the folder. Next, select the "Permanently delete old items" radio button. Click Apply and Ok out.

That's it! I told you it's easy.

I use these settings on "Deleted Items" and "Sent Items". One of my professors in College told me to delete old messages regularly, that way you can't be blamed for purposly deleting email to cover yourself, etc.