After a bit of testing, Motherboard said that some of the usernames did correspond to real Yahoo accounts, but that when they tried to contact more than 100 addresses, many were returned to sender as undeliverable, citing a disabled or discontinued account. While Yahoo investigates claims of the hack, it is recommended that readers with a Yahoo account act preemptively and change their password immediately. If Yahoo does disclose with the public that the hack really did take place, it's almost certain they'll get users to change their passwords too. Source: Motherboard via BBC News