I wonder if there's some sort of relay setup that could be used to mitigate this: e.g. if you want to run your own mail server, sign up for some kind of "mail ring" that transparently proxies the (encrypted) traffic from all the member mailserver administrators. That way, your public IP has a higher volume and more leverage getting delivered.
Integration with SPF and would be unfun and probably even impractical (max 10 lookups iirc?). I’d also worry about the inevitable abuse which would appear to originate from your range(s). If you can mitigate those though, it sounds interesting
