But it’s a free Europe-based provider that’s not US big tech. A better suggestion?

To be clear, I use a paid service (Mailbox.org) for my main email, as everyone should do.

They are right. Terminology is important in this discussion.

It’s considered a good idea because it runs over omnipresent, already-existent, distributed infrastructure. In other words, for this particular chat app, you don’t even need to create an account. That is at very least an interesting and noteworthy feature.

some issues with many email providers

This turned out to be the deal-breaker for me. GMX kept locking me out of my account because of the DeltaChat messages. They’re (of course) full of cyphertext and to email providers this must look a look like spam.

The open-to-abuse nature of email claims yet another victim.