I'm gonna add in Mailfence. I've recently switched to them (was thinking of Mailbox but had the same problems with them) and I've been pretty happy. I also use custom domains and it's very easy to set it up. They also allow you to generate custom aliases with their domain (e.g. "throwaway3661@mailfence.com").
The only kinda downside in my mind is the fact that you need the 42€/year tier to have access to POP/IMAP/SMTP and custom domains, which is more than on some other providers.
Another one I was looking at that looked quite good as well was Runbox, but if I remember correctly they don't have calendar and contacts which was a deal breaker for me.