The nation's largest broadband Internet service, which has more than 5.7 million subscribers, said it has implemented blocks based on subscriber accounts with the most outbound activity.
Port 25 is a gateway that most computers use to send e-mail. SMTP, or Simple Mail Transfer Protocol, which lets people send and receive e-mail, operates on the port.
Mitch Bowling, Comcast's vice president of operations, told XBiz that the company is singling out spammers on the company’s network.
"We don't think it's the right approach to blanket Port 25,” he said. “The right approach is to seek out people who are spamming our network and others."
Bowling said it the Philadelphia-based company is targeting accounts suspected of sending mass amounts of unsolicited e-mail, much of it pornographic.
Comcast estimates the daily email flow on the company's network at about 800 million messages, with only 100 million originating from its servers. The remaining 700 million came from so-called “zombie” computers, which distribute the spam.
E-mail virus writers have targeted Comcast to turn subscriber computers into zombies without their knowledge.
Network measuring site SenderBase estimates that 665 million emails a day come from Comcast domains, more than Yahoo and Time Warner Cable's Road Runner service combined.
Blocking Port 25 would prevent computers from sending emails from any non-Comcast SMTP server. This would shut down people using Comcast's bandwidth to send spam from SMTP servers. This would also limit PCs acting as zombies from connecting to mail servers outside Comcast's network.
Other Internet service providers have blocked Port 25. America Online and EarthLink have been doing this for many years, and other cable ISPs such as Cox Communications also have implemented Port 25 blocks to fight spam.