White lists are a drastic form of spam control, and only permit the delivery of e-mail to an account user if the address of the sender matches an address on a pre-determined list. This means that the user will be assured of not receiving e-mail from persons unknown to him, but will miss e-mail from anyone he has not added onto the white list.
The more usual anti-spam measure is the use of black lists, whereby the addresses of suspected spammers are identified, and only e-mail from those addresses are blocked.
In Hotmail's case, says CNet News, a user's Hotmail account will open on a "Today" page, on which e-mails passed by the white list for that user will be displayed. The white list will be made up of those listed in the user's contacts.
There will be a separate "Mail" section, in which all incoming mail, including that not passed by the white list, will be displayed. In addition, the upgraded system will also have a "Junk e-mail" section, for suspected spam.
Other ISPs, such as AOL and EarthLink, already offer similar features.