@reiver Erm... but that's not the issue with the email per se but the network and the sorry state of it.
In case of email it got monopolized by single entity (entities) which dictates the rules.
But - absolutely sam thing can/could happen with AP if ended up on the wrong path and there would be single biggest instance that can rule the whole network/protocol.
You said that it's very difficult to deliver email to gmail but even right now masto instances are banning themselves "just because" and I'm 200% certain that if at one point someone would decide to send mass spam from their self-hosted instance it would be banned to the ground.
It's just a matter of scale combined with ease of sending basic email (`echo "hello" | mail -s "your subject" your@email.com`) that resulted in more and more restrictions. However, with correctly set DNS (with SPF/DKIM) it should work just fine.
As for 7-bit/8-bit... yeath, that's true. But it is and will be true to any protocol that ages (SMTP is ancient) that has huge network that wants to maintain compatibility. It's quite possible it will happen to AP and distributed mastodon network. Right now its miniscule compared to email...