Officials said Friday they've contained another attack from the feared "SoBig.F" computer virus by identifying and blocking at least 20 "accomplice" computers in the United States and Canada.

An expert at antivirus company F-Secure said the virus that clogged inboxes this week with returned-mail messages would also download software onto infected machines Friday afternoon. The possibly malicious program would come onto infected machines through a connection with an encrypted list of computers hidden in the virus body. The list contains the address of 20 computers located in United States, Canada and South Korea.

Experts feared it could have deleted files, stolen passwords or created rogue servers for spreading junk e-mail, but an Internet security expert says all it did was visit a pornography site.

Friday afternoon, antivirus company MXLogic said that federal authorities have been successful in shutting down about half of the suspected IP addresses.

So far, SoBig.F hasn't been able cause any direct damage to an infected machine; However it can damage e-mail systems by sending out a high volume of mail to e-mail contacts. The worm takes an address from an infected computer's address book and uses that as the "from" line in dozens or hundreds of e-mails it sends out, making them appear more like legitimate mail.

That flood of "spoofed" e-mail results in returned-mail and "virus found" messages for users who don't have the virus.

If your e-mail program allows you to, you can deflect some of those messages by setting rules or filters for your inbox, funneling messages to your deleted items folder.

In popular e-mail clients such as Microsoft Outlook and Outlook Express, go to the Tools menu to set up the rules. In Outlook, look for the "Rules Wizard." In Express, look for "Message rules." You can elect to have messages with troublesome subject lines -- such as "Thank You!" "Your details," "Details," "Wicked screensaver" -- automatically deleted, or you can create a special folder the program diverts the mail to. That way, you can review the messages to make sure nothing legitimate is deleted.

Corporate antivirus company Postini -- which says it has processed 101 million email messages in the last 24 hours -- found that one in 19 messages was infected with a virus.

The worm is set to expire Sept. 10, but don't wait until then to delete it. Symantec has a program that you can download to remove SoBig, which lists itself on a computer as winppr32.exe.

How Are Viruses Affecting You?