Challenge/Response Spam Protection helps you to reduce the amount of spam in your Inbox.
If you decide to use Challenge/Response Spam Protection, anyone not included in your contact list or white list who sends you an email will receive an email containing a confirmation link.
The person, after clicking on the confirmation link, will be asked to retype a validation code.
Until the sender of an email confirms his/her 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 "Block senders" button.