When I was moving away from Fastmail, I noticed that Rainloop[1] is a pretty slick, Gmail-inspired webclient, and it's FOSS, but I couldn't find any provider who was actually using it. Can anyone comment?
I'm using Rainloop for my personal domains and so far love it. There's very good integration of GPG and (e.g.) Google Drive. Didn't take me long to move away from SquirrelMail or Roundcube after that. Highly recommended.
Their web client is the only one I've ever been able to use without wanting to quit email.