DKIM, SPF and DMARC all serve to prevent account spoofing, which isn’t a problem for Nostr and it’s priv/pub keys. The real problem is the opaque structure of mail providers, each with different spam filter mechanisms and preferential treatment towards “safe” origins (gmail, apple, microsoft, aws ses, etc…) encouraging a closed system
Yes, I agree. My analogy to SMTP was along the lines of "simple, open standard, with problems, but addressed over time due to things like DKIM. It doesn't need to be perfect from the start. Problems are solved over time". That's what I was trying to convey.