Domain registrar, cu.cc, got their whole domain labeled as spam due to misuse of their customers.
Miro from cu.cc asked why his whole domain was labeled as having malware when it did not have malware on it.
Google’s JohnMu replied to him in the Google Webmaster Help forums saying that subdomains can impact the parent domain. John said:
So when offering free subdomains and sites, be careful!
Forum discussion at Google Webmaster Help.