Large company? Literally every company I ever worked for, the smallest one were 5 people, had their own domain for emails. And they were not all in IT.

So we agree its insane to cut off every company with a domain? Good.

That would be insane, yes. It isn’t the case, though. I signed up with my own domain just fine.

I don't know what merchant system they are using but I will say it is very common for them to have their own anti fraud detection and rejection on charges. Your specific case could be related to a huge list of possibilities (even many of them a rollup of many other interactions on other sites that happen to use the same merchant or foundational fraud data).

Thinking this is just because of a domain name is silly.

Also Fly.io -- You may want to clearly accept and manage trial issues (or at least a subset) via a support path. It seems silly to effectively bounce users with the impression that you have no support because they are not YET paying customers while in the trial phase.

as a counter-point, i use custom domain for email, and haven't had a problem with fly.

It may not be custom domain but rather self-hosted mail server.

That's worse. A self-hosted mail server implies a significant cost investment that probably wouldn't be required for ye olde hotmail account.

No, we don't flag accounts as high risk based on email domain.