Mozilla Mail is a modern service... It is is distributed as Thunderbird these days. And while I have a lot of gripes with the way it's managed (ugh, don't ask!

Mozilla Mail is a modern service... It is is distributed as Thunderbird these days. And while I have a lot of gripes with the way it's managed (ugh, don't ask!) - it's still one of the best alternatives. Still sort of extensible if you sneak in the back way (i.e. not WebExtensions).