NOT OFFICIAL! After installing it (since it's a newer version than the regular one included in the current Software Manager), I ran into various issues. Nothing too major, but annoying: there were only English-language dictionaries, at least one plug-in was missing, no option to install a different icon theme; Clawsker didn't work. Hadn't had much experience (and much less knowledge) with Flatpaks, so I investigated. Shocked to learn that lots of Flatpaks are apparently EXPRESSLY unwanted by the original software developers (Claws-Mail team being one) and NOT supported. Also learned that those who make the Flatpaks often need to include absolutely everything anyone would want (in this case it should've had all language dictionaries, all icon packs and all plugins). There are many developers who are asking for the listings (like in the Software Manager) to remove links to the official software's site, because it makes it look more legitimate than it is. Note: this does NOT apply to ALL Flatpaks, some are fine and supported - easiest to check if you go to the official site and look for any reference to a Flatpak. Btw, now I'm back to the regular version of Claws-Mail and very happy. :)