But that’s not what the host default domain is. It has nothing to do with Postfix.
The hostname is just the final destination of mail sent to the server for any of the various domains that are hosted virtually in Virtualmin. That’s what the hostname is to Postfix. That’s completely unrelated to the “host default domain” in Virtualmin!
The host default domain has one purpose: To get a certificate so the user doesn’t get a browser warning when they log in.
It is not important. It’s not a thing you should be treating specially, and it’s not a thing that matters to Postfix. That certificate isn’t used by Postfix unless you force it to be, and you shouldn’t.
This is why I don’t like calling it “default domain”, because it makes people think it’s something it isn’t. It literally exists to request a certificate. After you have Virtualmin domains, you never even need to think about it. You don’t need to tell your users about it, you don’t need to use it when configuring mail clients, you don’t need to use it for logging into Webmin/Usermin. It is literally just a name to uniquely ID the server, and we’re guessing that for many users it will be a name that resolves so we can request an LE cert for it (and if it doesn’t resolve yet, no big deal, we just won’t get a cert and you’ll get a browser warning like you would always have up until a couple years ago).
The host default domain is for getting a certificate early in the install process. That’s it. That’s all it does, and all it’s for. Unless you make it something more (but there’s no reason to do so).
Not many years. All of the “host default domain” stuff has been around for maybe a year in various forms (maybe as much as two). We’ve had some disagreements about how it should be shown and whether it should even be in Virtualmin (though we all agree having a Let’s Encrypt certificate from the beginning is super convenient and helps users who don’t understand the browser warning about a self-signed cert).