Comcast sending out less spam

US cable giant Comcast has said the volume of spam originating from its network has dropped 35 per cent since it blocked an email loophole weeks ago.

The new data comes after Comcast, the largest broadband service provider in the US, earlier this month began blocking a gateway that spammers commonly use to send mass volumes of unsolicited email. Called "port 25", the gateway lets PCs send and receive email based on SMTP (Simple Mail Transfer Protocol), the most common technology for exchanging messages.

While a Comcast representative declined to offer an estimate of the service's daily email volume, independent measuring firm SenderBase confirmed the decline.

As of yesterday, Comcast's daily volume of email reached about 464 million messages, according to SenderBase. In contrast, approximately 665 million emails were sent out of Comcast's network on 10 June.

Comcast has been criticized for not doing enough to battle spam that originates on its network. Many spammers have taken advantage of Comcast's speedy broadband network to send millions of messages. Other ISPs such as America Online, EarthLink and Cox Communications have blocked port 25 for years.

In Comcast's defense, the bulk of the spam is the result of viruses that infect unknowing PCs and turn them into "zombie" spam engines. Blocking port 25 essentially closes the hole from which spam bots can disseminate their messages. The move is not considered an end-all to the spam problem, but it's an incremental step in siphoning off network abusers.

Despite the decline, Comcast continues to rank highest on SenderBase's email volume rankings. Web portal Yahoo ranks second with 297 million emails and Time Warner Cable's Road Runner third with 200 million daily emails.

Jim Hu writes for CNET News.com