I'd say that's lucky. I'm using "Masked Email" on Fastmail with a personal domain and I still find that I get blocked from signups from time to time. I assume they're looking for some sort of pattern?
In the alternative, you can configure Fastmail to allow sending and receiving to/from a wildcard address (which for sending allows you to specify the sending address at send time). In my experience this works far more reliably, and is one of the features that pushed me to move to Fastmail many years ago.