Google: Spam havening for a long time.
Google, and gmail in particular, has been home to 419 scammers for YEARS.
Regarding complaining to abuse@gmail, "we" have been doing that for years with no noticeable reaction (ie, no response and no cessation of the accounts being used). It's known in the spamfighting community as a complaint blackhole.
Google supposedly started cleaning up gmail's act a few months ago, but as I understand it their position when dealing with DNSBLs has consistently been "We are too big to block. If you blacklist us, people will stop using your DNSBL"
UUnet, Hotmail and Yahoo have held to the same position in the past (or threatened to sue).
Yahoo is still widely blackholed in part because of their sluggish/zero action about spammers using Yahoo mail accounts, but moreso because they happily host _PAYING_ spammers on Yahoo Stores.
Black helicoptor, because that's what's needed to find and eliminate spammers.