I don't think you can use Proof-of-email.
As mentioned before you can't prevent double spending, as it is valid for an email to have multiple recipients and besides that, email addresses are bound to domains. If you loose control of your domain name you loose your email addresses as well.
http://threatpost.com/phony-order-faxed-to-registrar-leads-to-metasploit-defacement/102576
This is a recent example of hijacking a domain by FAX...

interested