My account got marked as "high possibility of fraud", even though I validated my email and added a credit card they even charged. The error message said that I had to contact support@fly.io but that address only accepts email from paid subscriptions.
With no way to evaluate the service, I opted to delete my account.
Exactly what happened to me as well. In my case, I was (fortunately?) messaged by someone from Fly.io after asking around the community forum how to remedy the issue. I think the person I talked to was inferring that the system got tripped because I was using my own domain for my email address.
It's since been corrected, and I still have a handful of services running with them, but getting started with the platform definitely was a rougher experience than I was expecting it to be.
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.
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.
With no way to evaluate the service, I opted to delete my account.