Spam accounts for 80 percent of the e-mail arriving at Arlington, Va.-based US Airways, but filtering keeps all but half a percent of it from reaching its more than 28,000 employees' in-boxes, said spokeswoman Amy Kudwa.

Nucleus's Yadav said that these examples do not jibe with the experiences of most large companies.

"There may be specific examples where the IT administrators have set their anti-spam filters to very stringent rules," she said. "But because end users are becoming increasingly anxious about losing legitimate mail, some companies are becoming less aggressive in how they are setting their spam filters."

Yadav said companies underestimate the amount of productivity they lose to spam. Survey respondents estimated that spam was costing their companies an average of $220 per employee in lost productivity each year, about a tenth of the $1,934 that Nucleus estimated the companies were losing. Nucleus based the spam cost figures on an estimate of $30 per hour that companies spend on average to pay their employees' salaries and benefits.

Eric Hahn, chairman of Cupertino, Calif.-based anti-spam firm Proofpoint Inc., said he believes the spam problem is made worse because many companies and individual computer users are not using top-tier anti-spam filters.

"There's 300 vendors in the spam mitigation market, and 295 of them are not the best ... products out there," said Hahn, the former chief technology officer for Internet browser company Netscape Communications Inc.

Hahn said that companies should support stiffer legislation and aggressive law enforcement efforts to punish spammers. "More prosecution more jail time, better tighter prosecution will help," he said.

The federal Can-Spam Act took effect on Jan. 1, making it a crime to falsify the "from" and "subject" information in e-mail messages. Some states, including Maryland and Virginia, have instituted similar laws that carry longer jail sentences.