Try automating that for 15k sites and you have a whole new engineering problem in front of you!
It should be more or less possible though. Just not easy.
Just send a bunch of emails a day for days, for weeks, for months. It's perfectly fine if those compromised websites aren't ALL notified on the exact same day.
Just send a bunch of emails a day for days, for weeks, for months. It's perfectly fine if those compromised websites aren't ALL notified on the exact same day.