How to mark a sender as safe?

Select the mail address and click on drop down arrow and add to vip , it will marked with star icon and will indicate that this contact is a vip and safe in every sense , but it doesn't mean its safe the sender can send a spam mail to you in future ( as it's all good faith ) , but the contact can be added in address book .

Also address book is meant for saving contact details and not for filtering junk mail .

If you add them to your Contacts, they should be ignored by the Junk mail filter.