It's moderately difficult to guess (standard Fastmail masked email word.wordnnnn@fastmail.com), and additionally I emailed several of the other people on the sp

It's moderately difficult to guess (standard Fastmail masked email word.wordnnnn@fastmail.com), and additionally I emailed several of the other people on the spam and got autoresponders + real people confirming that they used that address for Comcast (and zero bounced email notifications), so I'm pretty sure that they didn't guess, unless they happened to be really good at guessing.