Users e-mails have not been delivered to their intended recipients as other ISPs block e-mail traffic from Tiscali.
Tiscali has not said how many of its 1.8m UK customers are affected by the outage which started eight days ago.
It said engineers were "taking urgent action" to block spammers using its e-mail service to send messages.
A statement on the Tiscali website read: "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. This is not affecting all delivery addresses. Some mail continues to be delivered as normal."
The ISP said it was currently installing new hardware and updating its spam filters.
"This work will be complete today. It is likely to take seven to 10 days for these changes to take full effect across receiving e-mail providers."
But some spam experts have expressed surprise that Tiscali has said its current problems are the result of spammers.
"There have been major problems in the past," said Richard Cox of Spamhaus, a not-for-profit organisation of IT professionals that tracks spammers and publishes free real time databases of offending addresses used by many big businesses and ISPS.
"Spamhaus is certainly not seeing anything that would justify major blocking, unless it is being targeted at one or two specific networks."
The organisation uses a network of anonymous e-mail addresses to "trap" spam.
"These are on most spammers lists so as soon as spam starts hitting those traps we know its happening," he explained.
Spamhaus does not use services such as hotmail, Yahoo or AOL for their spam traps so if these alone were being targeted they would not appear on the Spamhaus radar.
Mr Cox speculated that the problem could be the result of a technical failure at Tiscali, something the ISP denies.
Tiscali advises customers sending important mail to use an alternative free email service.