A normal company could engage in these practices, but it is settled law that a monopolist may not. A monopolist may not use their monopoly in one are to supress activities in another area.

You can't put these big tech monopolists into the same bucket as normal companies. They have way too much power, and even when they do not intend to smash things, they end up smashing things.

I wonder if we can’t build a far simpler postoffice app on http (self hosted domain space) and have a whitelisted encrypted exchange.

Substitute “HTTP” with “SMTP” and you already have that today… SMTP in plain text isn’t generally used anymore, it’s always transported over TLS. Of course you can run whitelist-only, but how is that ever going to work?

SMTP is text based and we’ll defined, so I would also argue that the transport being “simpler” is nonsense.

Whitelist is easier. If I added an mail of a friend in this system, we can communicate. Everything else goes blackhole.

Nothing is easy about email! Having worked for years just to get reliable in and outboxes is definitely not trivial. Also SMTP is a system out of your control if you want anything verified and actually delivered.

Of course whitelist is easier, but how valuable would your email be if you only allowed your friends to email you? Goodbye order confirmations, subscription reminders, recruiter emails, notifications…

Sounds like a dream, really. And you could always have it be user-driven action, like "Copy this line and paste it into your email provider's whitelist", framing it as a positive win for the user since they have total control over who is allowed to communicate with them.

Seems like you could easily do this yourself with a couple of rules in your email client.

I do want arbitrary people to reach out to me regarding my home page, so any just-allow-whitelisted-messages proposals are useless in that regard.

And in practice some minimal address obfuscation has been enough to block any address harvesters, and setting up a contact form or something comparable (and I guess especially also subsequently keeping it spam-free!) would have been much more of a hassle.

Yeah, I feel to that is a much better system. You could have a “grey”list postbox that uses a one time code for that company.

Say you enter a form at a company,and in the message box you just leave that code so they can mail you.

Of course we can, but good luck getting people to use it. They'll keep using what everyone else uses.