I like the idea of this in theory, but I am a bit confused about that happens in multi-email account senarios on custom domains.
As well as individual user accounts for individual family members, I also have a set of email addresses I use for different purposes. If I create a masked email address, say "example.mask0123@personaldomain.com, what email account does that come back into? The email account associated with 1Password? I can see that if I manually create a masked email when I am logged into
user@personaldomain.com through the Fastmail website than that would be linked to the masked address, but if I am using the 1Password integration on a website I don't want to use the account linked to 1Password, but "shopping@domain.com" for example.