If you held a Yahoo account in 2013, then you were almost certainly involved in the data breach. At 3 billion user accounts affected, this announcement sets the record for the largest known data breach.
https://www.notebookcheck.net/All-3-billion-Yahoo-user-accounts-were-involved-in-the-2013-data-breach.256130.0.html
So what? I don't understand why this would be a problem at all? Anyway Yahoo mail is full of spam and unimportant stuff.
Yahoo!
To kkdasld, these things are important because people's privacy are at stake. There are lots of important communications in people's mailboxes. Even if you have nothing important in your mailbox, you have the contacts of everyone you communicated with. That list is worth a lot of money for both legal and illegal purposes. Lots of people also use the same email addresses (and even passwords) as login credentials or two-factor verification purposes for other things. You gain one email, you gain other possibilities that are much more dangerous and nefarious. Things like TeamViewer, cloud-storage, remote-access, online banking... all the way up to identity theft are possible and have been done through credentials of a single email address.