So if you send email to the owner of the compane and to your colleagues on same level you put boss at last spot if they come to your mind as last?

Exactly. That's how I always did it and will keep doing it. Anything else sounds absolutely weird to me.

There is no mention of it being mandatory in my post

There kind of is. Because you make it sound like it's not a written rule per se but still very much a social convention that people kind of expect you to follow.

It's similar thing to as when we used to hold door to next person so they don't get smashed by it. boss > colleagues > intern.

So this is where it gets really weird. What is that even supposed to mean? How is the position in the company relevant to holding doors? If there's an intern behind me, I hold his door open. If it's the boss I do the same. Just like I'd expect both of them to do the same for me and each other. What is your list even supposed to mean? You don't hold doors for interns? Your boss doesn't do it for you? What kind of sociopathic hellhole of a system do you work in? Do you participate in that?

I once worked in a company where my supervisor demanded I'd prepare coffee for him and completely lost it when I wanted to discuss how exactly that was part of my job as a researcher. His point was that his boss had always expected his orders to be followed, no questions asked, and he'd expect the same thing from me. Needless to say I left that fucker as soon as I had another contract available (and never prepared his fucking coffee). This kind of hierarchical thinking may be appropriate for the army but certainly not for an ordinary company.