You want to delete unwanted email like Spam, it is easy with Automatic Email Manager. Here one solution which consists to delete all emails in your email box excluding some with one or more filters. Other solution is just to create a filter which delete emails which doesn't fit the rule.
To determine if safe email you can do it by entering the email list of safe sender, or any other rule, when done click next.
It will overwrite the account default action (delete).