I never quite understood what the value add was for email-client developers to use intermediary servers. Is it to gather usage telemetry or something else?
That being said, there are security/privacy implications to that model, and I wouldn't personally use an app that works this way. That's why Mimestream was built as a traditional client.
[1] https://stackoverflow.com/questions/41674063/is-it-possible-...
Note, this is in no way an endorsement of middle manning email management and facading as a client.