Something not mentioned much is that you can respond to these messages that come in through a Masked Email, and your identity is hidden on the outbound messages as well.
They seamlessly integrate with the sender identity feature in Fastmail making it very clear that you are replying from the Masked Email.
From a quick analysis on the headers, I don't see anything that leaks who your real identity is, but of course Fastmail knows and could reveal that if legal reasons exist.
Overall smooth feature along with the ability to use a custom domain for portability (to a less sophisticated wildcard setup, or another provider).
They seamlessly integrate with the sender identity feature in Fastmail making it very clear that you are replying from the Masked Email.
From a quick analysis on the headers, I don't see anything that leaks who your real identity is, but of course Fastmail knows and could reveal that if legal reasons exist.
Overall smooth feature along with the ability to use a custom domain for portability (to a less sophisticated wildcard setup, or another provider).