The go-to rich email client bundle is TBird with the openXchange bolt-on.

But I’d like to see what the teamz and oneScrote upgrade path is. As terrible as they both are, they’re entrenched; and a replacement needs to suck the same or less, while also protecting data sovereignty.

(Same or LESS, people. It’s a easy bar technically but workflow and UX aren’t considerations in the lost-boy post-dotcom world)