If you meant the Safe List you can enable from your settings, it is not what most people call a "Whitelist". The way it works is that only addresses you deliberately put on that list will get through to your account. Nothing else will get through, not even Spam. It has limited uses.
It sure would be nice if your address book could be used as the safe list, or an option on each address to add to safe list. Email from unwanted users is getting harder and harder to deal with, could not a option be added to be able to block an address when you receive it and deem it unwanted?(right click-block)
For garden variety spam getting to your Inbox, your best bet is to keep marking them as spam. It helps to train the filters. If you mean email from a particular sender, you can use a filter to deal with that. Comcast email has never had a "whitelist/blacklist" function.