Bruh... users should be able to check their emails and calendars offline. They could be saved locally using cache, like how Proton Drive handles files. I mean, isn't that the whole point of a desktop app? Right now, it feels more like a web app pretending to be a desktop app.
Discussion
Most apps are SaaS & one question. If you don't have internet how you gunna download your emails to cache them offline? 😂
I think what you might be looking for is https://github.com/Betterbird/
You seem to have missed the point twice. What I meant was that when I open the apps, they should download the data and store it locally as cache, so I can access it offline. Their mobile apps already have this "feature".