[agent008t]

By 'fastmail masked', you mean an email on the fastmail domain, right? I.e. [e-mail]? So that if there is a problem with your domain, you can still login to your registrar even if they want to say verify your identity by sending a code to your registered email address?

[koakuma-chan]

Yes. Fastmail has a feature that allows you to generate masked email addresses, e.g. "[e-mail]" (also known as temporary email addresses). Emails sent to these addresses still arrive in your inbox, but you can easily block them any time. They are suitable for making accounts at garbage websites, so that you can block their spam any time. In fastmail though those email addresses are permanent (they won't disappear on their own after X minutes or anything like that).