It used to be pretty easy to run your own email server, back when a lot of people did it. If someone is worried about a future where most cryptocurrency runs through a small number of providers, as email does today, I don't think they should find your comparison heartening.
I see what you are saying, I know SMTP fairly well, used to run my own server, and looked fairly deeply at DKIM / SPF / DMARC. However I also know blockchain protocols intimately, and I can say with certainty that the Bitcoin protocol and SMTP are completely different (as well as Ethereum, Monero, Stellar, Ripple, EOS, Tron, and on and on). It is just a completely different thing.
If you are worried about other wallets not accepting "my" wallet, as is the typical problem with hosting your own email, you don't need to worry. Money is money, if I receive it I receive it. It's just completely different from receiving a text-based message like the wide-open and free SMTP is.
