Yahoo has added a small but useful feature to its Web mail service that lets users filter the contents of their in-box to see only the messages from their contacts. This means that if someone's not on your contacts whitelist, you don't see their message.
Short of Yahoo Mail's built-in filters and its connections sorting, this is one of the simpler ways to cut out any in-box clutter from people you don't know. However, there's some work involved on your part to build that list of contacts. To enable the feature, users must first create a Yahoo profile over at profiles.yahoo.com. Then add people they wish to list as contacts.
One area where this terminology might confuse users is the difference between "contacts" and "connections." Yahoo Mail's help section refers to connections as "contacts with special status." In the case of mail, what makes them special is that you've interacted with them frequently, so the product assumes you know them. They must also confirm you as a contact before a connection is made. Contacts, on the other hand must be manually added, either through mail, or over on Yahoo's profiles site.
Yahoo Mail's senior product manager Rick Pal says this feature will only be available for Yahoo Mail users in the U.S. and Australia, and won't be rolling out to all accounts until a "few weeks" from now.
Update: Made a clarification on the difference between contacts and connections.