The number of directory harvest attacks (DHA) on corporate e-mail systems was the highest ever last month, according to security company Postini.
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DHAs are brute force attempts by spammers to find valid e-mail addresses, said Postini, which provides e-mail security management products to businesses.
Spammers guess addresses until they get some right. Those successful addresses are then harvested for use in later spam campaigns.
The fall-out of a DHA is that company e-mail servers are made to work flat out rejecting e-mails to incorrect addresses, which can affect overall corporate network performance.
"February was the worst month ever for DHAs," said Andrew Lochart, director of product marketing for Postini.
"In fact, the five worst days ever recorded by Postini were in February."
The average Postini customer was attacked 224 times per day with DHAs, with each attack comprising 166 invalid message delivery attempts, meaning there were 37,184 invalid delivery attempts per day on e-mail servers.
For February, the company said it protected customers from more than 25 million DHAs that attempted 4.2 billion invalid delivery attempts.