[klinch]

Now I'm wondering which of these extensions and strategies are employed by mainstream mail clients like Thunderbird, Apple Mail and Outlook?

I'm currently doing remote work from a location with an instable connection. Naturally I expected mails to work well in an async fashion, but instead... everything is really janky and I'm always unsure whether actions like moving an mail to a different actually 'went through' without loosing any mails.