Suggestion? Start now. I moved my primary email to a custom domain a bit over a year ago, and it takes a while to slowly migrate everything over. You don't want to be doing that while under pressure from whatever it is that forces you off.
They won't for some arbitrary percentage of emails. And even then, they will keep coming from new places.
The biggest thing is to get onto your own domain so that switching again if your host changes policies becomes much easier.
I use mailbox.org with no complaints.