Thanks for the response. Just to be clear, my post was less a call for synchronization functionality, more an effort to understand how you intend the feature to be used across multiple devices in its current form.There is no cloud storage for privacy reasons, so this isn't possible and given the goal of the project also not desirable.
Also, the app is for 98% free to use, so paying servers for cloud storage wouldn't be feasible. The usual way is to show ads, track you and analyze your behavior and sell the acquired data to earn money. Fairmail will never do this.