I use mailbox.org and have no issues with it. I don't think the web interface looks amazing but I only use it with IMAP clients so it's not an issue for me. You

I use mailbox.org and have no issues with it. I don't think the web interface looks amazing but I only use it with IMAP clients so it's not an issue for me. You can get a free trial account which is quite limited in functionality but will show you the web UI at least, so you can make your own decision. I have also used soverin.net and broadly found it was fine, but ran into a couple of issues like difficulties managing spam settings, poor documentation on website etc. Worth noting that soverin.net also requires a phone number to sign up, which some people dislike on privacy grounds (may or may not be a deal-breaker for you).