Directory harvest attacks (DHA) have shot up over 500 per cent, according to new figures.
The attacks, where hackers send out emails with randomly generated names to valid domains in order to find out if an email address exists, rose by 505.6 per cent last year, new research from security company Sonicwall found.
The company said that it recorded twice as many of these attacks than all spam attempts.
The company said that image-based spam had also increased massively over the past year. This type of spam recorded a 500 per cent increase in 2006, while 'pump and dump' penny stock hype scams rose by around 400 per cent last year.
"Overall, we've seen a growth of 274 per cent in unwanted email during the year, almost half of which is spam, but there are many ways of halting the flood," said Gleb Budman, senior director of email security at Sonicwall.
He said that one of the easiest ways for an organisation to combat unwanted email was to check that its email filter was properly configured.
"Companies sometimes have their own domain on their list of allowed mail, which creates an open door for spammers," he said. "Stacking multiple different email systems typically catches only a few extra spam attempts, but will double the amount of good mail mistakenly caught, thereby increasing the complexity of management."
The company also found that there was a 64 per cent increase in the numbers of definite phishing emails.
