Email in itself is decentralised. You can set up your own email server and send and receive from it. However, your emails will be often flagged as spam unless you perform some form of black magic and allegedly, you are more vulnerable to spam (although I have been running my own email for months now that’s on the open web and hasn’t been spammed). Some hosts like Hetzner and various ISPs don’t allow you to open the ports needed for email, at least without giving them a ring and explaining why you need it.
I understand what you mean, having had to deal with SPF nonsense for a job back in the day. The SPF nonsense is what prevents selfhosting email, as you effectively point out yourself. If there were somehow a way to use federation to tackle that, it seems like it could be kinda cool.
