Spam messages jumped to a record 94 percent of all email sent last month, according to research from Postini.
More than 25 billion spam messages were blocked last month, an increase of 144 percent from December 2005.
This rise in spam is linked to a virus released by hackers in late December, called the "Happy New Year" worm, which infected high numbers of computers with botnets and then pumped out spam, experts at the email management firm said.
The worm, also known as Nuwar and Mixor, was hidden in electronic postcards posing as greetings from friends and family. This widespread attack drove the daily number of email-borne viruses up 20-fold over the New Year's weekend.
"This continued rise in spam levels is threatening the viability of email for businesses that are not properly protected," warned Daniel Druker, executive vice president of marketing at Postini.
Ken Dunham, director of the Rapid Response Team at VeriSign iDefense, said late last month that the "Happy New Year" worm, containing malicious variants of the Tibs, Nuwar, Banwarum and Glowa malware, was employing more than 160 servers to mass-spam email users.
Sophos also reported last week that the top virus of December was the Dref mass-mailing worm, which, although first seen in July 2005, was spammed out as a New Year's e-card. The virus accounted for 94 percent of all infected emails on Dec. 31, according to the anti-virus vendor.