Interesting concept. It's not common but I have seen it at sites like
PayPal. Might not be a bad addition to auth. As mentioned above, one
approach may be to use list:string for the auth_user email field. Or maybe
add a field for "secondary emails" since you probably want one email
address to be "primary". But if you are going to want to store information
about each email address (for example, suppress marketing messages), you'd
probably need to create and email address table with references to
auth_user.
