Every X/K/Ubuntu Member gets an @ubuntu.com email address. Their Person.$name is used to generate the email address. This email address forwards emails on to their preferred email address (as we don't provide POP, IMAP, webmail or even storage facilities).
Currently, you can set your preferred email address to <email address hidden>. If you do this, you are saying that your <email address hidden> email address should be forwarded to <email address hidden>, which is very broken.
We have already had at least one instance of a user setting their preferred email address to their @ubuntu.com one.
I suspect we need to special case this somehow - probably involving a new email address type of ALIAS, which would be an email address which we know redirects to another one in our database with a CONFIRMED or PREFERRED status.
Or just refuse to allow @X/K/ubuntu.com email addresses into the EmailAddress table, maybe listing the @ubuntu.com email address on the relevant screens for people who have signed the code of conduct. I say maybe as we might not want to advertise these until PersonNameBlacklist is implemented.