Comment by jspash

HBO Max sounds like a big company (I've heard of HBO, and "Max" sounds big to me). But it is possible they are using something like Mailchimp for their mailing list and don't do it in house. I'm just guessing here - a quick look at the headers would reveal this.

And with a confusing and horrendous UI such as Mailchimp's, it's quite easy to send a test email to the "live list". VERY easy indeed.

We've done it twice now. Once to about 10,000 emails and another to almost the entire list of 800,000. Luckily the template we were testing was 95% complete and not many people noticed. It just looked like the email got truncated with gibberish at the end.