[mdiesel]

But on the flip side, good luck making any kind of sensible Gmail account for example, which means either fastmail needs multiple domains or their service will see new customers reducing over the years as people can't get sensible email addresses.

The same thing is true of phone numbers, but it's more obvious that there's a finite number of... numbers... So they need to be reused eventually.

[nicky0]

Fastmail does indeed use multiple domains. However it's much better to use your own domain with fastmail to truly own your email address.