Your domain name could be ordered to be removed from US-based dns providers, no matter which TLD it is. That would essentially block your website from most US-based viewers without actually shutting down your hosting. Advanced users could still get to it, though. Consider hosting through Tor and a .onion address for more resiliency.

Look into https://simplelogin.io/

They make creating random aliases for custom domains like this easy.

As for the domain name itself, anything that already looks like a mail service is good. “examplemail.com” or “mailexample.com”