The bigger and more concerning issue would be recipients complaining about the incoming email and marking it as spam, which could damage the reputation of HBO Max's domain and potentially send future legitimate messages to spam. (provided the integration tests don't use a non-primary domain for sending)

I learned this the hard way in my last role, I worked for a small company that wanted to do custom email marketing. I was pretty gung-ho about it, I thought I'd just set up a script to loop through contacts and use mailgun to send the email from a custom domain. As we used the tool we saw a steady drop-off in click-throughs to the site, the majority of our messages were getting caught up in spam filters.

Turns out there's a whole science to email marketing, how emails should be structured and formatted etc. A lot of times the criteria for spam is how often the domain was flagged for spam in the past, the length of the email, the contents etc.

Ended up relying on this tool quite a bit: https://www.mailgun.com/deliverability/email-spam-checker/