I've used this service for three years. I used the free version for a couple months and then upgraded to the paid plan. It's awesome. The support is very responsive as well the 1 time I had an issue. And the problem wasn't on their end. I have 1 domain for my email on Mailbox.org, and another domain with MX setup for addy.io. So when I want to make a new email I just type one in on the fly. Don't have to create one on the site or extension first.
I can't talk about their ownership structure. Seems to be one person or maybe a small team. But that's just a guess.