They have their code open sourced, and you can self host it if you want. I personally did not bother since I do not use their service for outbound mail (which would definately get marked as spam) and I trust them for now. It also has a neat temporary mail plugin (which I put under a subdomain) that might come in handy sometimes.
Not answering the question directly, but since it allows you to use an arbitrary service it might be relevant.
I'm on the free service and have had no issues. I was nervous about it when I started: https://paperstack.com/moving_parts/
I have no affiliation with ImprovMX. I should probably upgrade to the paid service though now I think about it!