Is there some sort of web interface for self-hosted e-mail that is reasonably good and sleek/modern-looking like Gmail, and also works well on mobile with swipe gestures and everything?
But then I also very much prefer a hierarchical/directory tree approach to organizing e-mail than labels&search as Gmail does it.
More similar UX-philosophy can be found in Mailpile[1] and Cypht[2]. Both still have decent amount of moving parts but are continuously progressing.
[0]: https://github.com/roundcube/roundcubemail/
[1]: https://github.com/mailpile/Mailpile
[2]: https://cypht.org/