At its worst the phenomenon can even wipe Internet servers off the map.

Last month, Stephen Gielda, president of Packetderm, upset a fraudster who was trying to use his anonymous Internet service. Soon his servers were inundated with a tidal wave of backscatter messages. At one point, he was being hit by 10,000 bounceback messages per second, enough to throttle the server's Internet connection.

Gielda had to take his site off-line for five days as he waited for the problem to abate. "I'm used to backscatter, but I'd never seen it at this level before," he said.

While backscatter is extremely hard to filter out, it is a problem that can be fixed.

Backscatter comes in three varieties: messages from mail servers, saying that there is no such user available; "out of office" automated reply messages; and so-called challenge-response messages, which tell the sender that his message will be delivered only once he responds to the bounceback and confirms that the e-mail is coming from a legitimate address.

Security experts say that people should simply stop using these last two types of bounceback messages.

As for "no such user" bouncebacks, that can be fixed too. There are a few e-mail standards that could help with the problem: Variable Envelope Return Path (VERP) and Bounce Address Tag Validation (BATV), for example.

But the problem would largely disappear if server administrators configured their mail servers to immediately reject mail that is sent to nonexistent users, rather than accepting it and then bouncing it back to the faked addresses. Some ISPs (Internet service providers), AOL for example, have done this and have largely eliminated their role in the problem.

If there is spam in the backscatter message, antispam software should filter it out, but if a message has an ambiguous subject line, like "Hey" and the spam message stripped out, the backscatter will look like a legitimate bounceback and is probably going to get through, said Dmitry Samosseiko, manager of Sophos Labs Canada.

"This is a serious problem that is hard to deal with, to be honest," he said. "We can blame spammers for causing the issue in the first place, but it exists because of the mail servers that are not configured to deal with spam."