You can block spam at a couple different points: on the mail server, in your case Yahoo/SBC/ATT, and also on your computer system if you are downloading your mail into a local program such as Microsoft Outlook, Outlook Express or Microsoft Mail on Vista.

For your SBCGlobal/Yahoo account you can log in to your account and on the top, right-hand side you will see a link with a drop-down for “Options.”

Select “Mail Options” from the list and this will display the “General” page.
On the “General” Page select “Spam” from the list in the left-hand column.
On the “Spam” page you can adjust your settings in both the SpamGuard and the Blocked Email Addresses sections.

Make sure the Control SpamGuard box is checked. See the other options for your own needs. You can also add specific email addresses to the Blocked Addresses list.

Yahoo Mail and the mail providers such as SBCGlobal & ATT also offer customers the option to upgrade to their SpamGuard Plus service which offers additional spam-blocking options. Check with your service provider (AT&T/SBC) for specific details on features and price.

Remember, one of the key problems with spam beyond the annoyance factor is the possibility of your system getting infected from a malicious email. See the post on Internet Security at: Keep Your System Protected