Hopefully this is a learning lesson for everyone, recently swapped from zapmail to an inbox provider that offered a much cheaper model (subscription + domains, inboxes are free).
I've come to learn that over the last 4-5 weeks, the deliverability issues my campaigns have experienced is entirely on them, because the sending IP is burned (verified by checking the reason emails bounced from instantly).
Let this be a lesson to not be cheap and let it be more expensive.