The number of e-mail messages infected with viruses rocketed in May, with an increase of more than 380% on April, according to managed e-mail service provider Postini.
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Postini said it had intercepted nearly 184 million virus-ridden e-mails in May, with more than three quarters of these carrying the Sober worm – a level the e-mail service provider described as “staggering”.
In the four days between 3 and 7 May, the percentage of e-mails infected with Sober – 14% – was higher than the percentage of legitimate e-mails, at 13%.
Last month also saw a sharp rise in the number of directory harvest attacks (DHAs) by spammers against corporate e-mail.
DHAs increased by 30% on April’s figures, with more than 33 million attacks – an average of 243 a day against every customer, Postini said.
Tuesday 3 May, immediately following the May Day bank holiday, was the worst day for DHA attacks, with more than 1,620,000 recorded, just as the Sober worm infections hit their peak.
The top three viruses logged by Postini in May were Sober, with more than 142 million infections, mytob with around 14 million and netsky, with close to 11 million. The bagle virus, which has sparked recent warnings, hit fewer than 890,000 e-mails in May.