Also, how do you mask your identity if you self-host? I can have as many mailboxes as I want but they're all trivial to correlate because they share a domain that isn't providing email accounts to large amounts of users. And then there's the matter of a VPS not actually being under my control. It's a VM running in a datacenter. I could run the mail server locally, but then I'd still need to relay through a VPS to mask my IP address. And that's still only protecting from a casual adversary...

What do you mean by "masking your identity"? If you self-host at home, then your IP will be discoverable through DNS, but no one but the ISP will know who the account holder is. Registering a domain also normally requires providing a name and address, but no ID is normally required and it is an open secret that a large proportion of WHOIS information is fake.