In this case, according to federal prosecutors, AOL insiders. From the San Francisco Chronicle:
An America Online software engineer stole a list of 92 million customer screen names that was eventually used to send massive amounts of e-mail spam, federal prosecutors said Wednesday.
Jason Smathers, 24, was arrested at his home in Harpers Ferry, W.Va., and was charged with conspiracy. Smathers, working at AOL offices in Dulles, Va., stole the list and sold it to a Las Vegas man, Sean Dunaway, who used it to promote an Internet gambling operation and sold it to spammers, a criminal complaint said.
Dunaway, 21, also was arrested at his home and was charged with conspiracy.
Each man could face up to five years in prison and at least $250,000 in fines if convicted.Dunaway reportedly charged $52,000 for the full list, or $2000 for each letter of the alphabet.