Well the solution to old email protocols is JMAP, which Fastmail supports. But it's going to probably take time until JMAP support really picks up with both mail providers and client developers.
Based on what I've read from RFC8620, their approach is quite smart. It's basically turning email protocol into a set of HTTP RPCs, thus shifts all the transport-related technical problems away.
The downside is that, JSON codes creates redundancy, sometime the redundancy is bigger than the meaningful data itself. But on the other side, you can now host an email server behind a web CDN. I like the development overall.
