I know online services or any service depends a lot on personal interaction experiences and personal expectations from those services and people involved in those services, but I can't leave mailbox.org sooner.

I have tried/explored couple of them like Mailbox.org (of course, current user), Fastmail, Proton, and Runbox etc. Given everything http://runbox.com seems to be the best among them - much better than Mailbox. In fact for Mailbox their less than 'less than ideal' support is enough for me to move elsewhere once my balance runs out (which I foolishly pumped in more than I should have had)

Runbox isn't w/o its challenges but they do certain things really nice and they are: prompt, not into disregard a request in its entirety, not un-kind, not-jerk, not-flippant, not-entitled, not-costly, not into offering unusable shit that you don't need in an email suite to begin with, and basically open and engaging.

So while I am not a customer right now, once the mailbox.org balance is close to running out and I will leave mailbox (and leave them I will) Runbox is the first contender to replace them right now.