But with Fastmail you can already do it using $RANDOM_STRING@$LOCAL_PART.fastmail.com and whichever password manager you use (you just have to do it in a more manual way, which in the long run can be a PITA, I understand)
One of the big selling points of this idea, and why we wanted to partner with 1Password for it, is that it has to be easy! Sure creating a new password for every site is a good idea, but the 1P plugin makes it so easy that it's simpler to use the really secure password it generates than to come up with something yourself.
And this "generate a Masked Email right there in the form where you're using it" pattern means that the friction is so low that it's a viable choice - it's EASIER to do the safe thing, and that's the real game changer.
Absolutely, making it easy it's a big selling point. As a happy FastMail customer, I would really like to see a similar thing with BitWarden which is my password manager of choice.
