Filtering spam messages is a thankless job for software. For every 100 spam emails, one message usually gets through, an irritating pitch with links to Web sites selling questionable drugs or sketchy Rolexes.

<A HREF="http://www.infoworld.com/article/07/10/31/Researchers-dig-for-hidden-links-in-spam_1.html?source=rss&url=http://www.infoworld.com/article/07/10/31/Researchers-dig-for-hidden-links-in-spam_1.html">The links contained within spam</A> are one indicator in determining whether it should be blocked. Often after a large spam run, the addresses of spammy Web sites will be added to blocklists that are used by antispam software to cull future messages with those links.

To get around it, spammers construct emails with links that can't be identified by filters but still are valid in the messages, said Christopher Fuhrman, a professor of software engineering in the Department of Software and IT Engineering at the University of Quebec.