I think Mastodon and other fediverse servers already require that your ISP allows outgoing email, since there would be no way for you to send your password reset or account verification emails

yeah this is true, but if you were to build a whole system around emails, then the contents would be arbitrary and the magnitude would be far greater, increasing the chance of running into problems with spam blacklists which ISPs hate to be on (see, for instance, digitalocean's spam policy)

And deliverability to providers like Gmail is not a big deal to me IMO either, if say Raddle can suddenly talk with people from riseup.net for example, that's a big win to me.

yeah this may or may not be an issue depending on exactly how the system works

Anyway you're the actual dev here who has way more experience doing server shit than me so maybe I'm just spouting some BS lol, so I will just defer to your wisdom :)

i don't think it's BS. i just foresee there being some problems with the idea, and i know from experience having worked with mail servers that it's something i personally never want to touch lol. but if this is a thing you want to build, then i think you should go for it, if only for the learning experience.