Freedom2Surf developed the solution in-house and it is based on collaborative open source systems and references spam blacklists maintained by a number of organisations such as Spamhaus.
The emails are therefore blocked if they originate from a server identified as a source of spam.
The company believes this was an important basis on which to develop the system -- rather than using more invasive and less effective methods such as keyword filtering.
By rejecting on the grounds of the sender's IP address, the company claims it protects users' privacy by not having to scan the contents of emails.
Chris Panayis, managing director of Freedom2Surf, said: "The service stops the most heavy spam abusers from getting through to people's inboxes, even the spammers that frequently hop from ISP to ISP to evade detection."
There have been growing calls for service providers to start blocking spam at the ISP level and users will be hoping this move will be the start of a growing trend.
Panayis said: "The ISP industry has a responsibility to help protect their customers from being exposed to and having to deal with spam on a day-to-day basis. We would urge other ISPs who have not already done so to partner with anti-spam organisations to develop similar spam-filtering systems."