I'm sorry but this is really a nothing burger, a pure nitpick.
Who gives a fuck where they host their customer support emails?
You can spin up new mullvad accounts with 0 friction. Even if you doxx your account in an email, you can create another one.

I don't know any service who "self hosts" email for customer support. That's probably a terrible idea since self-hosted email often gets flagged as spam and that's the last thing you want for customer support.

Your note makes it seem like this is some dealbrealer and that "they don't give a shit about your privacy".

I don't think you have bad intentions but you're letting the perfect be the enemy of good and the result is that people who read that will just be discouraged and not improve their setup.

I think we should be vocal about things that matter. A customer support MX record pointing at gmail is not it.