[throw10920]

They're Fastmail masked email addresses, of the form [e-mail] - so you can enumerate the address space, but I would hope that they guard against that, and I got two spams within a short period of time to the same address.

[nonasktell]

they don't https://email-checker.net/validate [e-mail] exists [e-mail] doesn't exists

most mail providers don't guard against that

[geocrasher]

Have you asked Fastmail about it, then?