Yahoo was massively hacked two years ago and that at least 500 million customer records have been stolen. Yahoo admitted that security questions and answers have also been grabbed by the hackers. BT is currently investigating the Yahoo data breach and is advising those who havent changed their passwords post-December 2014 to change them. You should also ensure that you are not using the same password anywhere else on the web (password reuse is a cardinal sin) and that you have enabled two-step verification on as many of your online accounts as you can.”]
Source: https://grahamcluley.com/investigates-yahoo-hack-tells-yahoo-mail-users-reset-passwords/