i found this in yahoo mail's help----
What are filters, and how can I set them up?
Filters automatically sort your incoming messages into folders that you create or alert you of new email on your mobile device. They do so according to rules that you set up. The benefit of filters is that you don't need to read through each message to decide where it goes. For example, you could create a filter so that messages from your family members are automatically delivered to a personal folder called "Family," or messages from coworkers trigger a text alert on your mobile phone.
You may create up to 15 filters in your free Yahoo! Mail account. If you need more filters, check out Yahoo! Mail Plus. One feature of the Yahoo! Mail Plus service is a limit of 50 filters.
To create, edit, or delete a filter, click the "Mail Options" link on the top-right navigation bar, and then choose "Filters" located on the left side of the page, under the Management column.
You can also add a mobile alert for a particular sender by clicking on the "add mobile alert" link in the message header of the message view. This link will prompt you to register your phone to activate this service. Once you register, the link will pre-populate the filter page to enable you to easily add a mobile alert for this sender.
Creating a New Filter
To create a new filter, click the "Add Filter" link on the Filters page.
1. Choose the field you want to match in the incoming message. For example, select the one of the following headers: From:, To:/Cc:, Subject:, or Body to match.
2. Choose the criterion by which you want a match to be made. For example: contains, ends with.
3. Enter the text string to compare. For example: @rocketmail.com, Love, InfoBeat.
4. Choose the destination folder to which you would like the message delivered. For example: Personal, News, Admin.
5. If you have more than one filter, you'll also see "up" and "down" arrrows to the right of the filter list. Use these buttons to select the placement of your new filter within your existing sequence of filters. Filter order is important because Filter 1 will be the first filter applied to an incoming message, Filter 2 will be the next, and so on until a match is made or all filters are passed. If no filters match an incoming message, it will be delivered to your inbox.
Be sure to click the "Add Filter" button to put your new filter into effect.
Editing or Deleting an Existing Filter
Once you reach the Filters page, click radio button to the left of the filter you would like to edit and click the "Edit" button located on the top-right side of the Filters page. After making the desired changes be sure to click "Add Filter" for your changes to start taking effect.
To delete one of your filters, click radio button to the left of the filter you would like to delete and click the "Delete" button located on the top-right side.
