Do ‘unspoofable’ email protocols exist?
There are several protocols actually:


SPF Sender Policy Framework is used to verify that the e-mail address used to send the message is valid e.g. if the sender had rights to use the given domain.
Recipients can verify this by checking the ips of the agent dropping the mail against the allowed ips listed in the domain DNS records.
DKIM DomainKeys Identified Mail is used to verify that a given message hasn't been tampered in transit.  Recipients can verify this using the public key of a given domain along with the signature included in DKIM signed e-mails.


These are actively used in SPAM filtering.  You can see it in action by looking at the headers of most e-mails today.  You can look for the headers: Authentication-Results, DKIM-Signature, Received-SPF.