Where is the key kept then? One possibility, the user has to know it, at which point we're back to the fact that users dont seem to want a password for their email app (again, happy to see an interesting post on the generalities of email app security). The other approach is to store it somewhere on the phone, at which point connecting the phone to a computer as you describe is still an attack vector; you just need to find the key.
Of course, I am not highly versed in security, so if there's another option I'm interested to hear it.
So therefore, your article could have been titled "{Mailbox|GMail|iMail|all_other_mail_clients_ever} is a Security Fail!"?
Because as far as I am aware, few mail clients either support or (if they do) actively encourage an extra password layer, and your users do not want it. Given an average un-password-protected phone, you will be able to read their email even if they were using the iOS encrypted files framework, just by opening the app.
I apologize, but it appears that your headline is deliberate sensationalism. If you want to have a discussion about how we need to secure email apps in general, I'm interested. If you want to just pick the latest 'big thing' and take pot shots at it, nah.
Of course, I am not highly versed in security, so if there's another option I'm interested to hear it.