For emails (or any Outlook item) you will not need beyond today, don’t just delete them, Shift+Delete them.
Simply deleting emails will send them to the Deleted Items folder. Then, you will have to remember to empty the Deleted Items folder. Not to mention, for those connected to an Exchange server, items in your Deleted Items folder count towards your mailbox quota. Instead, use Shift+Delete to get rid of them.
Please note that Shift+Delete is a PERMANENT solution. There is no retrieving an item or email that has been Shift+Deleted. Make sure you really want to get rid of the item before pressing Shift+Delete.