Spammers and other abusers are constantly attacking and testing networks. If an ISP's abuse team is unwilling to listen to outside complaints and take them seriously, then testing will be missed and perhaps develop into full-scale abuse, perhaps a botnet. Now that's expensive for an ISP. The flip side of port 25 blocking is the Spamhaus PBL or Policy Block List. Absent special arrangements between a user and a service, the user ranges at consumer ISPs can be said not to be legitimate sources for SMTP traffic. The PBL is a list of such ranges that recipients can block wholesale, and then put in exceptions as warranted.And since this is a chance to take a dig at it, I'll note that my own ISP, verizon.com, is listed at No. 9 and is the source for the infamous Gevalia coffee spam. I actually think the spam abuse flood currently sweeping over Google caught the company by surprise, as it did Yahoo and Microsoft in their day. Think of the work it must have taken for Microsoft and Comcast to dig themselves out of this hole. Of course, Microsoft may be No. 11 and I wouldn't know, but Cox said Comcast, in recent years, has become "an impressively proactive ISP and they stomp out a lot of abuse as soon as their people are aware of it." So ISPs really can turn it around if they're willing to do the right thing. There's no doubt in my mind that the work an ISP does to eliminate this sort of abuse from its network will also improve the quality of experience and support for users, especially its own, but also outsiders. Security Center Editor Larry Seltzer has worked in and written about the computer industry since 1983. For insights on security coverage around the Web, take a look at eWEEK.com Security Center Editor Larry Seltzer's blog Cheap Hack.
And it's not just spam that gets ISPs on lists like this. The Spamhaus Top 10 also reflects hosting of spam URLs, fast-flux DNS servers and other abusive practices. Look at the Spamhaus complaint list for Google, for example, and you'll see more than one incident related to hosting of spam URLs on blogspot.com. There are also many complaints about docs.google.com being used as a spam redirector.