An integration test is a type of software test that makes sure different parts of a system work properly together.

So HBO has some integration tests that make sure their system can send email properly. The way it does this is by sending some test emails to a test email list.

Except somehow, someone made a mistake today and the test email got sent to the real email list of all the subscribers.

So my best explanation is I am a YouTube TV user and I saw that there giving a 5 day trial for HBO Max. I think the integration is between the two companies so we can test it out for free.

I think that email was them flipping the switch and forgetting to remove the test email. This is my hypothesis as a seasoned Server Engineer.