Re: Let me count the ways...
First up, of course emails are not private - that's laid down in the spec.
I'd argue that private correspondence can still take place using postcards. Just because someone might overhear or see what you're saying, doesn't necessarily make a one-on-one conversation public.
What 'personal data' is there?
Even if the email is encrypted, there is still personally-identifiable metadata - sender email address, mail client headers, IPs, etc.
What's more, it would be absurd to argue that the recipient is the one sending data to Google - that is, obviously, what the sender has done. Again, that's by definition.
Sort of, but only indirectly. If I send a postcard to a PO Box in Bristol, and the person managing that PO box has instructions to forward everything to an address in Kentucky, did I send that postcard to the US? Or did the post office do that, at the behest of the PO Box owner?
Email is somewhat similar: When most people send an email to email@example.com, they firstly hand it off to their ISP or email provider, whose email server checks the DNS of smallshop.co.uk for where to send the email (ie. smallshop.co.uk are instructing the email server where to forward the message). If that destination server is in the US, well then.