ET3D: I agree that it does seem likely that there was a hack, but trying to get accounts one by one is also not too far fetched. I occasionally get notifications of someone trying to get into my accounts at various places. I also imagine that people reuse passwords and that can easily be used to attempt to get into their accounts. Any account that doesn't always notify you of a wrong login can be a good target for brute force, and most places work like that.
CharlesGrey: But millions of accounts at once? And I'm fairly sure major services such as Google or MS do have protection against such plain brute force hacking attempts.
Once the salt is figured out, calculating hash checks for passwords would allow you to bulk check the 200,000 most common passwords and find instant weak passwords for emails.
However if you don't use a common password, or variants, and if they require a unique salt per user (
say, the email is also part of the salt) so your password would perhaps convert to: emailservice.saltemail.password.salt2, which means you can't brute force millions at once (
on a single machine), unlike in other systems with lower security where it would be closer to: salt.password.salt2, and calculating a single hash could be checked against all accounts for an identical match.
ET3D: Depends on the size of the botnet.
Tens of thousands or millions of computers unknowingly working together to crack passwords...