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I almost hope the data is made somewhat public so Troy / https://haveibeenpwned.com/ can get a hold of it and provide the public with reassurance.

By now I suspect you can simplify it down to just matching on the RHS for any domain registered to Yahoo.

A number of ISPs have used Yahoo to provide mail services in the past, so it's probably not quite as straightforward as that.

And not just small ISPs, but major ISPs, such as BT (with 32% of the broadband market-share in the UK).

That was my first thought. I used to have a Yahoo email and I'm assuming at this point (multiple hacks), it's out in the wild.

I used to as well but if you still can access it you should delete it: https://help.yahoo.com/kb/account/close-yahoo-account-sln204...