Can't imagine how frustrating this must be for Customer.io, but probably also for the former FoundationDB team, especially that FoundationDB is now acquired by Apple and probably can't respond (or would have to get special exceptions).
We used customer.io for the past few months but due to bugs with the system, we switched to sendwithus with sendgrid as our ESP. We've been really happy with the results.
Hey there! Glad you found a solution you're happy with. If you didn't let us know about the bugs you experienced, would you mind sending me details - colin at customer.io ? I'd love for us to have the opportunity to fix them for the future.