As mentioned in a recent post about abuse of URL shorteners, Yahoo! is currently a popular choice for spammers wanting to host their ads on a 'trustworthy' domain. Spammers create Yahoo! groups or profiles, post their ad copy to the profile page or as a message to the group, and then send out spam containing the relevant URLs. Because the URLs contain the 'yahoo.com' domain name, they aren't good candidates for URI DNSBL filtering.
Our traps have been picking up a lot of this kind of spam recently, so I decided to try to work out how big the problem really is.