> How would you establish trust in your domain from a tor exit node?

Presumably, you wouldn't? But just because you can't establish trust this way, doesn't mean you couldn't maintain it that way, does it?

> So you can't build trust into your domain without getting a both a trusted domain on a trusted TLD on a trusted IP.

Yeah, so what? You can't upgrade an operating system without installing it first ... therefore you can't upgrade it? What's your point? Maybe you still need a "static IP" to gain reputation. Maybe someone finds a different solution for building reputation. Why should we keep one unnecessary restriction just because there is another currently necessary restriction? Maybe someone builds a sort of "new domain reputation escrow system"? You pledge 1 bitcoin (or whatever) that gets donated to charity if a panel of judged from the internet community decides that you used your domain to spam, and in return you get a reputation boost in a public database that email servers can check? Who knows, there are endless options for solving that second problem, which really have nothing to do with the first problem.

> The identity issue is not of concern

Erm ... you do understand that without identity, there can not be reputation, right? Even if the only thing you do is that you trust emails coming from a particular IP address because of good experiences with emails coming from that IP address, that only works because the IP address provides identity. The equality of client IP addresses between two SMTP sessions is the only thing that allows you to come to the conclusion that "this is the same entity that didn't send spam last time". This is all about identity and identity only.

> since there is no good way to tell if the IP of a domain's mail server changed to a tor exit node because it was compromised, sold or otherwise maliciously claimed or if the owner legitimately did it.

There is no good way to tell if a domain changed ownership to a spammer even if the IP of the mail server does not change. There is no way to tell if an IP address that didn't spam before changed ownership to a spammer. There never is a way to just know that some identity has not become a spammer. The involvement of a tor exit node is completely irrelevant to all of this.

You never know whether the next connection from an identity with good reputation is spam. You can't. That just isn't how reputation systems work. The point of a reputation system is that it creates an incentive to protect your identity against abuse because otherwise your identity will be muted.

> Lots of people will therefore treat changing the endpoint for mail as a new identity (for good reason).

Well, I can't tell whether "lots of people" do that, but it's certainly a terrible idea. If you have a business relationship with another company, and that company happens to move their email server to a different provider, how would it be anything but a completely braindead idea to penalize their emails for that in your spam filter when the emails are authenticated via SPF and DKIM as still coming from the same domain, and in at least that case thus also from the same company?

> These go for both IPs and Domains. SPF and DKIM are a great way of avoiding trashing your reputation because some spammer is sending under your domain. But we already have tools for reputation.

Yeah, and those tools build on SPF and DKIM ... your point being?

> If you build an IP reputation database, you'll quickly find out that Home IPs will get trash reputation regardless of if you think they should be allowed to send anything.

What is your point? I am not even sure if this is supposed to be an objection?! I mean, a lot of what you write isn't wrong, but just completely besides the point. I suggest that positive domain reputation should override negative IP reputation, and in response you point out that dynamic IPs have negative reputation ... when that is exactly the reason why I suggest that positive domain reputation should override negative IP reputation!?

And no, of course the fact that I think that domain reputation should override IP reputation does not influence the reputation of dynamic IP addresses ... why would anyone think that it would?!

> (That's not even mentioning that Home IPs aren't stable)

Again: Your point being?