Delta chat's chatmail public servers are mostly blocked. When trying to use email services like Yandex mail or Rambler you'll immediately get permanent ban for suspicious activity or spam (they know about PGP and Delta chat of course). The most popular Mail_ru gives you an external app password only if you pay for subscription, you can't use delta chat unless you paid them and even if you pay them you likely will be banned for "spam". So you can't just tell people to install delta chat and be connected - relay by default blocked, local email services doesn't want to see your encrypted messages...

Nostr-based messaging is much worse than anything you can use in such conditions. The number of DM relays are less then fingers on one hand. Dms here are superbad, it is unusable from my view. I changed client (now I'm using Wisp) and my old dms are missed, and those somehow appeared doesn't looks good. Nostr uses too many connections, it depends on availability of relays and good internet channel. I often struggle to send some basic event to three of the most popular public relays. I don't think Nostr can be the solution somehow. It failed in many ways. I tried White noise by the way, 0xchat too, idk it always lags and you have to admit compromises or surrender on it.