NEW YORK - The holiday season brings festive parties, family gatherings, and a deluge of spam.

Unsolicited messages, or spam, which account for nine out of 10 emails, fill up the inboxes of computer users more than ever at this time of year, experts say.

"Every year we see a seasonal increase around the holiday season. It's just worse than it's ever been before this year," said Daniel Druker, executive vice president of marketing at Postini, a company that provides message security services.

Spammers spew out millions of emails. Some hawk pharmaceuticals and sexual aids, others offer hot stock tips.

The unscrupulous commit identity theft by luring unsuspecting recipients into disclosing personal information, while others commit fraud with the lure of phony offers.

The glut of spam can clog business communications systems to the extent that emails at the workplace can be held up for hours, if not days, experts say.

"The threat of this is that email becomes no longer productive as a tool, and that is scary because email is ubiquitous. Most businesses could no longer run without it," Druker said.

Spam cost an estimated US$17 billion ($24.73 billion) in the United States last year in lost productivity and the expense of measures to fight it, according to San Francisco-based Ferris Research.

Worldwide, the cost was estimated at US$50 billion.

Around the holidays, spammers take advantage as people use computers for online shopping, experts say.

"You see a big increase around the holidays of messages where they are trying to fool you into buying things or trick you into providing your ID," said Jerry Upton, executive director of the San Francisco-based Messaging Anti-Abuse Working Group.

"People are busy and I think the abusers take advantage of that," he said.

The amount of spam has exploded in recent months, experts say, as spammers have adopted new tricks. Research by Postini found a record 93 per cent of email was spam from September through November.

"The spammers are definitely winning at this point. It's gotten much worse," said Gerald Thain, a professor of consumer law at the University of Wisconsin-Madison who specializes in email issues.

One of the newest dodges is sending spam in the form of an image rather than text, allowing it to get past filters that trap spam by hunting down specific words.

So-called image emails account for some 30 per cent of junk emails, compared with just 2 per cent in 2005, Postini said.