Section 3.10 of RFC8620 scares me. I know a lot about that and a lot of the complexity around that trite point is missing.
Note I've designed and built a complex messaging system that runs over HTTP over unreliable connections and run it in production for 15 years so I know what can and does go wrong.
Have you looked at their list of what doesn't work offline? It seems like it's mainly attachment management limitations than limitations of managing the emails themselves.
Note I've designed and built a complex messaging system that runs over HTTP over unreliable connections and run it in production for 15 years so I know what can and does go wrong.
Popcorn at the ready.