How scary is this?

You might want to change/double check your accounts

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The alleged malicious hacker who first came forward with the details of the Yahoo accounts — said to include usernames, hashed passwords, birth dates and backup email addresses — told VICE Motherboard last month that the Yahoo data is from “2012 most likely.” Read more here.

Review your Yahoo Mail settings
Delete email contacts that you don’t recognize.
Delete linked Mail accounts that you don’t recognize or control.
Change your password on any linked accounts that you control.
Make sure your vacation response is turned off.
See if someone else has been accessing your account.