If you decide to use the Challenge/Response Spam Protection, then anyone not included in your contact list or White List who will send you an email will receive an email containing a confirmation link.
Until the sender of an email confirms his email by retyping the validation code, the email will be stored in the "Unverified" folder in your mailbox.
Here you can approve or block the senders by selecting the email(s) and clicking on the "Approve senders" or "Blocked senders" button.
Email address of senders you block will be added to the Black List and emails from this address will be detected as spam and moved to the Spam folder next time.
Email address of senders you approve will be added to the White List and emails from this address will not be detected as spam and moved to the Spam folder next time.