Thousands of Tiscali customers are demanding answers after hackers broke through the internet service provider's security to send spam causing the email service to collapse.

The spammers evaded the Italian firm’s security system to send junk mail, which led to many other ISPs blocking all emails from its users.

Angry Tiscali customers have left messages on the telecommunications company’s website demanding to know why the company failed to inform them that there was a problem, which first arose last Thursday. Engineers are now “taking urgent action” to block the hackers, according to Tiscali.

“We have been targeted by spammers using our service, which in turn has meant other ISPs have taken measures to block mail sent by Tiscali,” a statement on the company’s website said. “This is not affecting all delivery addresses. Some mail continues to be delivered as normal. This work [installing new hardware and updating spam filters] will be complete today. It’s likely to take seven to ten days for these changes to take full effect across receiving email providers.”

In the meantime the ISP is advising customers to temporarily switch to an alternative free email service.