Tiscali will take at least a week to resolve problems with its customers' emails, the ISP admitted on Wednesday.
The problem arose because spammers set up free Tiscali webmail accounts and used them to send out large volumes of unsolicited emails.
On Friday of last week, Tiscali noticed a surge in the number of complaints from customers saying that emails they were sending were not being received. It soon became clear that other ISPs had started blocking emails originating from Tiscali's SMTP servers, effectively shutting down many Tiscali customers' outgoing mail.
An update from Tiscali on Wednesday said its engineers would install new hardware and update spam filters by the end of the day.
But Tiscali also said it could take seven to 10 days for other ISPs' mail servers to recognise these changes and start accepting mail from Tiscali users again. In the meantime, Tiscali suggested that customers use "an alternative free email service" if they wanted to guarantee delivery.
A company spokesperson said on Wednesday that the estimate of seven to 10 days was a "total hygiene prediction", and claimed that mail was still getting through to some ISPs. The spokesperson added that, once Tiscali had cleaned up the spam problem, it would perform forensic diagnostics to identify which accounts were being used to send the spam and shut them down.